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

a. Degler does more than merely summarizing
b. Degler has done more than a mere summarizing of
c. Degler does more than merely summarize

Here, I prefered merely over mere as I want to modify (summarizing or summarize) so b is out.

but cant choose between a and c

Kindly help.