The logic behind the question is that there is no correct answer, but Option B is more likely to be reflective of the decoy effect.

Consider these 3 decisions:

Decision 1: Option A: $350 / 70 migraines avoided Option B: $100 / 50 migraines avoided

Decision 2: Option A: $350 / 70 migraines avoided Option B: $100 / 50 migraines avoided Option C: $100 / 40 migraines avoided

Decision 3: Option A: $350 / 70 migraines avoided Option B: $100 / 50 migraines avoided Option D: $500 / 70 migraines avoided

There is no right or wrong answer to Decision 1, but if you choose A on Decision 1 then you should also choose A on Decisions 2 & 3 (since A & B are the same options, and C & D are clearly not better options). Similarly, if you choose B on Decision 1, you should choose B on Decisions 2 & 3. So responses to all 3 questions should be the same.

But it turns out that people who get Decision 2 are more likely to choose B than if they'd gotten Decision 1, because the presence of C (which is easily comparable to B, and clearly worse) makes B look better. And people who get Decision 3 are more likely to choose A than if they'd gotten Decision 1, because the presence of D (which is easily comparable to A, and clearly worse) makes A look better. This is called the decoy effect (or the attraction effect, or asymmetric dominance).

The ideal way to test this would be to divide people into three groups, and give each group one of these 3 decisions. But, failing that (with everyone taking the same survey), we can also just give everyone Decision 2 and guess that, if one subset of people is more likely than another to choose B, then they are more susceptible to the decoy effect. (Or, we could just give everyone Decision 3 and guess that, if a subset of people is more likely to choose A, then they are more susceptible to the decoy effect.) It's not a perfect design, but it is evidence.

(It's also possible that the difference arose because people were using the reasoning that Nick_Tarleton describes in his comment, in which case the question was tapping into something different than what it was designed to test.)