First: the question at the top has been transcribed incorrectly. Follow this link (thanks uymba!) to read the correctly transcribed question:
https://www.beatthegmat.com/retirement-t ... 25376.html
Second:
please DO NOT post questions for which you cannot look at the original source AS YOU ARE TYPING IN THE QUESTION. Sorry to go "all caps" on you, but this is
critically important. Everyone is now trying to figure out which one is the correct choice, and re-writing their notes and driving themselves crazy... and it turns out that the correct choice isn't even there. Obviously, that's not an effective way to study!
Third: karmayogi, nice summary of when to use "so." Think of "so" as similar to a pronoun, except that instead of replacing a noun, it will replace either a verb or an entire clause (verb + other words). Here's the (correctly transcribed) original sentence:
A study on couples' retirement transitions found that women who took new jobs after retiring from their primary careers reported high marital satisfaction,
more so than those who retired completely.
The "so" refers to "reported high marital satisfaction" (a verb, some adjectives, and a noun).
Each group reported a different level of marital satisfaction; the "so" allows us to refer to that concept without repeating all of those words.