C
Polygon's have many angles and:
poly = many
gon = angle
The selection that contains a fragment is B.
Every other sentence contains two distinct clauses put together by the use of a relative pronoun (which in A and C) or a conjunction (so in phrase D). In B, the two clauses "she doesn't give herself enough credit" and "that's too bad" are not linked syntactically and are simply juxtaposed.
Although not grammatically "correct", the use of a fragmented syntax a frequent trait of oral speech.
You should present more evidence
I hope this helps!