The conclusion of the argument is as follows: one who aids others primarily out of a desire for praise does no deserve praise for that aid. Why? Because a desire for praise is the desire to obtain, as a sign that one is good, the favorable opinions of others. But, people merit praise only for those actions motivated by a desire to help others.
This is a Strengthen with Sufficient Premise question. Remember that a sufficient premise is sufficient for a conclusion, if and only if the existence of the premise guarantees or brings about the existence of the conclusion. Therefore, we need to find the premise that 100% guarantees the conclusion. The way you want to attack these answer choices is two-pronged. Ask yourself, does it strengthen? If it doesn't, then cross it out and continue to the next answer choice. If it does strengthen, however, then ask yourself whether or not the premise 100% guarantees the conclusion.
The argument tells us that if people deserve praise, it is for those actions motivated by a desire to help others. Let's diagram:
DP ==> MDHO not MDHO ==> not DP
Thus, the only information in the passage that is sufficient to lead us to the conclusion "does not deserve praise," is "not motivated by a desire to help others." We know that a desire for praise is the desire to obtain the favorable opinions of others. But, being motivated by the desire to obtain favorable opinion of others does not necessarily exclude the possibility that it is also motivated by a desire to help others.
Answer choice (A) is the correct answer because it both strengthens and guarantees the conclusion. It eliminates the possibility that being motivated by the desire to obtain favorable opinion of others could still mean the possibility that it is also motivated by a desire to help others. So, (A) tells us it cannot be both. Thus, since we know that a desire for praise is the desire to obtain the favorable opinions of others, it cannot be that these actions are also motivated by a desire to help others, leading us to the conclusion that those who do this do not deserve praise. Not only does this answer choice strengthen the argument, but as you can see, it guarantees the conclusion by invoking the sufficient condition of the contrapositive of the principle diagrammed above.
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