Conclusion:The happy life tends to be a morally virtuous life.
Premise: genuine happyness consists not in pleasurable feelings but instead in one's sense of approval of one's character and projects.
It's Nessesary Assumption question so be on the look out to new information in the conclusion, which is "a morally virtuous life". The right answer should close the gap between "a morally virtuous life" and "one's sense of approval of one's character and projects. "
So the Answer choice B is correct.
I found the conclusion of this question unusual cos it introduces new information "good life" and give an explaination of it while refutes another alternative explaination. The key to find the right answer is to focus on the argument core.