• By going via the “Dustbin 1NT” to bid 2♥, South has shown (95%) six hearts and a weak hand (six or seven points); with a better hand, he’d bid 2♥ straight away. The 2♥ bid is not a suggestion in a two-way conversation, it is virtually a command for opener to pass. Hand (c) is the right answer.
• With Hand (a), you were correct to bid 1NT, but should now give a preference back to 2♠ - bear in mind that partner has shown a five ♠ - four ♦ shape. Going back to 2♠, rather than leaving partner in 2♦, is best because (i) it gives partner a chance to bid on and (ii) a 5-2 fit (♠) normally plays better than a 4-3  fit (♦). Don’t even think of bidding 2♥ - partner will pass holding a singleton heart and you certainly don’t want that.
• With Hand (b), you also must not bid 2♥ over 2♦ - risking playing there facing a small singleton heart. You can hardly go back to 2♠ and Dustbin 1NT hands do not bid 2NT next time (that would show 10-12 points, which you cannot have). That leaves Pass – there’s nothing else.

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