Which South hand fits the bidding nest?

Solution

Without investigating any further South bids 3NT. Which hand is best suited for such a bid?

South 1 is the perfect hand for the bidding as shown. South knows there is no fit in a major suit and has an excellent (double) club guard. By bidding 3NT he also correctly shows his point strength: opening strength, but not much more, say 12-15 points.
(Aside: can North now make a slam try? The answer is yes, but only just. His 1♠ bid neither denied nor showed extra strength: it showed 12-18 points. Only holding 19 points would he have jumped to 2♠ in the second round of bidding... if he has five diamonds that is, for such a jump shows 5-4 at least in the bid suits. However, North cannot have 15-17 or 18-19 points and a balanced hand, since he would have opened 1NT or rebid 2NT respectively. So if he has 15-18 points, his hand is unbalanced. If he has 17-18 points he could make a slam try - just).

South 2 does not know enough to warrant the 3NT bid. A 5-3 fit in hearts has not been ruled out yet and if there is such a fit, 4♥ is the contract to play, not 3NT. Therefore, over 1♠ South 2 should bid 2♣, fourth suit forcing, asking North for more information. A good agreement is that North now prioritises bidding his three card heart suit before showing a club guard.

South 3 realises 3NT is indeed the best contract* but he should realise as well that presumably North should play the contract. After all, aces do an excellent job in dummy, while North's honour cards probably work better fourth in hand, especially the ♣Q, but also the ♥Q and the ♠A.
Therefore South 3 should bid 2♣, the fourth suit forcing, as well, trying to make his partner bid notrumps first:
- If North bids 2NT South raises to 3NT*.
- If North bids 2♥ (three card heart support), South repeats the fourth suit by bidding 3♣. North will probably bid 3NT on as little as ♣Qx, meaning the contract is in the right hand.
- If North bids anything else, South has no choice but to bid 3NT himself.

(*Although if North has a long diamond suit and some good honour cards, 6♦ may be on:

However, South 3 cannot bid 3♦ - forcing, slam try -over 2NT, since this would show a four card suit. So he has to settle for 3NT, knowing there is a slight risk of missing a slam.
The message is: holding opening strength 'only' and three aces opposite a partner who has opened, you may consider a slam. Three aces make a hand stronger than the 12 HCP suggest!).