Apopulation of caribou lives on an isolated island located on the arctic circle. there has been no migration in or out of the population in the last 200 years. the recessive allele frequency for spotted fur has been 0 percent for 50 years. there was recently an increase in the recessive allele frequency. what is the most likely reason for this increase?

I encountered this question before but it had choices. The missing choices of this question are:
A- immigration 
B-Natural selection 
C- Random mutations 
D- Geographic isolation 

My answer would be C - RANDOM MUTATIONS.

A. Immigration is not applicable because it states There has been no migration in or out of the population in the last 200 years.

B. Natural selection is not applicable because of it states that the recessive allele frequency has been 0%.

D. Geographic isolation is not applicable because it is not a factor in the increase in the recessive allele frequency.