I am not sure about your question, but as far as SC is concerned, here is my approach:
1. ...stable food resources and sedentary lifestyle.. , these 2 permitted, so plural verb is required. A and E are out
2. in B,
at the foundation..., it's not at
A. increasing population,
was the foundation of the Mesoamerican civilization that followed -
Singular verb - incorrectB. increase in population, were at the foundation of the Mesoamerican civilization which followed -
usage of word at is not correct.C. increased population, were the foundation of the Mesoamerican civilization that followed
D. increased population, were the foundation of the Mesoamerican civilization that had been following -
it's not like civilisation had been following -incorrectE. increased population,
was the foundation of the Mesoamerican civilization that had followed -
Singular verb - incorrectSo by PoE, C is correct.