Shreyas Iyer, Hanuma Vihari, neither, or both? Of all the questions that will keep India's team management awake on the nights leading up to Boxing Day, this one might be particularly vexing.
All four options could come under consideration, with Ravindra Jadeja out of the tour, making a five-bowler combination trickier to balance, and with Ajinkya Rahane's form potentially putting his spot under doubt.
The case for Iyer is simple. He got his opportunity against New Zealand, and grabbed it with a hugely accomplished 105 and 65 on debut. The Test match was in the balance each time he came in to bat, and he steered India to positions of strength both times before being dismissed. He's already close to being a fixture in India's white-ball teams, and his domination of domestic attacks - he has a first-class strike rate of 80.22 from 56 matches - has long suggested he belongs at a higher level in red-ball cricket as well.
That Iyer got that chance at all, however, was down in part to the selectors leaving Vihari out of the squad and putting him on an India A tour of South Africa instead. If that decision was aimed at getting Vihari ready for the Tests in South Africa, it could be argued that he has readied himself most excellently, with scores of 25, 54, 72*, 63 and 13* in three four-day games in Bloemfontein.
Just as much as it was a demonstration of ability in South African conditions, however, it was a demonstration of ability against South Africa's second-stringers. While Vihari was scoring those runs, Iyer was scoring runs against New Zealand's Test attack, except he was doing it at Green Park and the Wankhede rather than Centurion Park and the Wanderers. And as skillful as Iyer's displays were, he was acing an entirely different test to the one he will face if picked in South Africa.
Take, for instance, his first-innings hundred in Kanpur. On the second morning, Kyle Jamieson delivered an uncharacteristically wayward spell, and Iyer took full toll, clattering him for six fours in five overs. Up to that point, however, Iyer had faced 32 balls from New Zealand's seamers and scored seven runs off them. Against their spinners, he'd scored 68 off 104.
Iyer, of course, was only doing what generations of India batters have done to visiting attacks: play the waiting game against the quicks, and milk their spinners ruthlessly. Mayank Agarwal did much the same during his 150 in Mumbai: 45 off 119 balls against the fast bowlers, 105 off 192 against the spinners.
Both innings were masterclasses in punishing minute errors in length and line from spinners in subcontinental conditions. But neither Iyer nor Agarwal will get too much of a chance to unveil that side of their game in South Africa. The last time India toured the country, fast bowlers delivered 91.34% of the balls South Africa bowled in the series.
Unless illness or injury intervenes, Agarwal will open India's batting at Centurion, and will immediately face a ruthless examination of his technique against pace. He seems to have narrowed his stance since his struggles on the 2020-21 tour of Australia, but it can't be said conclusively if that's helped him overcome the technical issues that dogged him on that trip. On day one in Kanpur, for instance, he was beaten and squared up multiple times by Jamieson before he nicked off, shaping to go forward but not quite managing to transfer his weight fully onto his front foot in time - the same issue that had caused him strife in Australia.
This isn't to say Agarwal won't make runs in South Africa - just that, in order to do so, he will have to check boxes entirely different to the ones he checked against New Zealand at home. The same holds true for Iyer.
What of Vihari, then? Eleven of his 12 Test matches have come away from home, and in them he has shown plenty of evidence of his quality against fast bowling - a half-century on debut at The Oval, 93 and 111 against a good West Indies attack wielding the Dukes ball, a counterattacking 55 against New Zealand's short-ball tactics in Christchurch, a match-saving 237-minute vigil on one leg in Sydney - but with an overall average of 32.84, he hasn't made himself indispensable.
You could argue, though, that he's only ever played in challenging conditions and against strong pace attacks. In the 11 away Tests he has featured in, Vihari's average of 34.11 is better than those of Pujara (34.00) and Kohli (32.11), and not a whole lot worse than Rahane's (38.94). The one time he got to bat at home, India were already 436 for 5 and looking to declare. Tough cards to be dealt. But then again, there's no saying how Vihari would have done in Iyer's place in Kanpur or Mumbai.
And so, back to India's middle-order conundrum at Centurion. Iyer, Vihari, neither, or both? A difficult question, to which there are no right or wrong answers.