Which mechanism decreases the number of genetic variations in a population? a founder's effect
b. mutation
c. gene flow

Founder's effect

The founder effect happens when genetic diversity declines due to an external larger group of organism invades a set of current living population. An example of this process is when the British colonies invaded the Mesoamerican continent where native americans lived and were outrun by the British colones, especially since many wars were made and ethnic cleansing. 

The mechanism which will decrease the number of genetic variations in a population will be called gene flow - c.

When we're talking about gene flow, we're referring to the 'drift' of organisms from a certain habitat to another habitat where they would increase the number of genetic variations for the same reason - sheer presence in a certain area.