The population of the world these days ... very fast. a-is increasing b-was increasing c - increase d - increased

A. is increasing

Explanation:

The phrase "these days" tells us that we want something in the present. If it wanted the past, it would say "in those days" or something similar. So we can cross out d and b, because they are past tense.

It can't be c, because the subject-verb agreement is wrong (it would be increases, not increase).

We can conclude that the answer is a, The population of the world these days is increasing very fast. It's currently increasing.