Thanks for the question. I'll start off by listing the premises in this problem.
P1) Desire for praise is desire to obtain favorable opinions of others P2) Merit praise - > Actions motivated by a desire to help others (remember that "X only if Y" is diagrammed X - > Y)
Conclusion: Aids others primarily out of a desire for praise - > ~Merit praise (Recognizing that merit and deserve are used as synonyms here)
Now we need an assumption that allows us to draw the conclusion. Before looking at the answer choices, based on our diagramming, it'll probably have something to do with linking the desire for obtaining favorable opinions of others with actions motivated by a desire to help others. This is what (A) gives us. (A) lets us conclude that (using the contrapositive of P2)
Desire for praise - > Desire to obtain favorable opinions of others - > ~Motivated by desire to help others - > ~Merit praise
