No flaw. (A) presents a conditional relationship: if firm crust, then baked at the right temperature. It then validly concludes that the absence of the necessary condition leads to the absence of the sufficient condition.

(B) presents a conditional relationship: if a cake tastes good, then it has the right amount of flour. It then invalidly concludes that the absence of the sufficient condition (not tasting good) leads to the absence of the necessary condition (not having the right amount of flower). The stimulus also invalidly concludes that the absence of a sufficient condition leads to the absence of a necessary condition, so (B) matches.

Wrong flaw. (C) presents a past trend (no winners being under 30), then invalidly concludes it will continue to be true in the future. The stimulus, meanwhile, invalidly concludes that the absence of a sufficient condition implies the absence of a necessary condition, so (C) doesn’t match.

Wrong flaw. (D) gives a single similarity between yeast and baking powder (they both cause sweet rolls to rise), and then invalidly concludes that they must be similar in another way (yeast can replace baking powder in sweet roll recipes) The stimulus, meanwhile, invalidly concludes that the absence of a sufficient condition implies the absence of a necessary condition, so (D) doesn’t match.

No flaw. If there’s less competition in cake contests when compared to pie contests, contestants will generally have a better chance of winning a cake contest.