Most moths of a particular population died when the chestnut blight fungus that they were dependent on got wiped out. but some moths did survive.
which mechanism explains the reason for this?
gene flow
genetic drift
natural selection

The correct answer is -

Genetic drift.

Explanation:

Genetic drift can be described as an evolutionary process that occurs by chance event, by which frequency of gene variant (also known as allele) within a particular population changes drastically.

In other words, it is a random phenomenon, which can lead to significant change in the allele frequency of a population.

As per the given information in the question, most of the moth population (that were dependent on blight fungus) died when the chestnut blight fungus got wiped out completely. Only few moths survived.

This will result in the drastic change in the frequency of allele within the population of moths.

Thus, it represents the phenomenon of genetic drift.