“more so than” is an idiom

It is used in making a "more than" type of comparison where you also want to refer to a previously stated verb/action (that's where the "so" comes in). In this case, the "so" refers to "reported high marital satisfaction."

Notice here that you are comparing two groups of women: those who take new jobs, and those who retire completely. When making a comparison, you have to make sure you explicitly mention both groups.

B, C, and D don't do this. (In fact, B and D change the original meaning of the sentence to make the second part of the comparison a hypothetical - but the original meaning is that the ones who take new jobs are being compared to those who don't.)

Also remember that, for comparisons, the two things/groups / whatever compared should be parallel. "women who took" and "those who retired" are parallel. The other options aren't.