Sentence Correction

[spoiler]Source:Egmat OA is C[/spoiler]
The Colorado settlements, whose year-round stable food resources and sedentary lifestyle permitted increasing population, was the foundation of the Mesoamerican civilization that followed hundred years later.

A. increasing population, was the foundation of the Mesoamerican civilization that followed
B. increase in population, were at the foundation of the Mesoamerican civilization which followed
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
E. increased population, was the foundation of the Mesoamerican civilization that had followed

My question here is "Is the term increased population correct. If so why and what does it mean?"

Because of subject verb agreement, only B,C and D can be considered.

B. increase in population, were at the foundation of the Mesoamerican civilization which followed
B looks Good. is B wrong?

C. increased population, were the foundation of the Mesoamerican civilization that followed
Mesoamerican civilization THAT followed means that there are other Mesoamerican civilizations too?
Permitted is the verb that Increased (adjective) Population (noun)?

D. increased population, were the foundation of the Mesoamerican civilization that had been following
Had been following is incorrect.