plural or singular???

One question from Kaplan.

The institutional nature of the town meetings in many New England communities serves as a ready-made forum for discussion of topics ranging from......

(A) The institutional nature of the town meetings in many New England communities serves as

(C) Institutionally, the town meetings in many New England communities serve as

(D) The town meeting, an institution in many New England communities, serves as

My quesion is, for answer C and D, according to the unlined part "a ready-made forum", here we should use singular "meeting", while according to " in many New England communities", means there should be many meetings rather than only one meeting "in many New England communities". Please help me to explain my confuse. thanks a lot.

then what's the difference between C and D, the best answer is D. thanks a lot.