South has:

What should he bid?

A 3♥
B 4♥
C 4♦

 A 3♥ (21 votes)
3♥ is a severe underbid. South will bid 3♥ too if he has zero points. After all, he is under obligation to bid.
In view of this possibility (very little with South), partner North will now pass if he has little or nothing in reserve (like 16 points and four cards in one or both majors). Meaning NS will almost certainly miss a good game.

B 4♥ (6 votes)
A lot better than 3♥, at least NS end up in game. However, 4♥ is by no means certain to be the best game since partner may very well have four spades and three hearts (if he has extra strength he can have a non-ideal distribution; even a doubleton hearts is possible then).

C 4♦ (5 votes)
4♦ is the best bid. South conveys two messages at the same time:
- ‘I want to be in game’.
- ‘I have at least two suits; please choose’.
North will now almost certainly bid a major suit, after which South of course will pass.
South could have bid 4♦ holding for instance 4-5 in spades and clubs. Over North's 4♥ bid South will then ‘correct’ to 4♠. North then has a choice between ‘pass’ and 5♣.