Aconclusion includes a concluding statement that the reader cannot ignore.
a commonplace
b comfortable
c comfortable
d unchallenging

The answer would have to be D, unchallenging due to its context. The author is describing a final word that is described as something that is obvious (the reader can’t ignore it).
Therefore, within that context, the adjective would have to be something that “grabs” the readers’ attention. That would obviously rule out commonplace, which, by name alone doesn’t catch you as something unable to ignore. Oddly that answers B and C are the same, but can also be ruled out quickly within context. “Comfort” also suggests that it is almost redundant, but certainly not something to keep an audiences’ attention.
The only answer, therefore, that answer that makes sense within context, as well as providing an eye-keeping ending for a conclusion paragraph would be unchallenging.