(b) is correct.

• After your 3♦ jump support showed around 10-12 points with four(+) diamonds, partner’s 3♥ was primarily stopper-showing, looking for 3NT. Clearly partner is weak in either spades or clubs, or he’d just have bid 3NT. Your 3♠ shows a spade stopper and inferentially denies a club stopper, otherwise you’d just have bid 3NT.Therefore Hand (b) is correct. 

• Hand (a), on the other hand, should bid 3NT, with stoppers in both unbid suits.

• Hand (c) is too strong for 3♦ in the first place (3♦ is a limit bid, which partner can pass). A better response is 1♠ - 100% forcing.