Well, I also got it worng- I also choose A ...

But if you look closely at the following part of sentence-

Degler does more than merely summarize existing research; he constructs a coherent picture of two centuries of studies dealing with the changing roles of women.

Parallel structure shld be maintained

You can see through parallelism in this case, but that is not really the rule. The original statement is grammatically incorrect in its own right.

To use 'summarizing' you need to be doing it still (Present Participle or Continuous)-
"I am summarizing"... "He is summarizing"... "to be summarizing"

"He does more than merely summarize" is the proper form here.

ex. I do teach. I do exercise. I do watch.
What did you do? teach, exercise, yoga
Present Participle - What are you doing? teaching, exercising, watching