Hardy-Weinberg Principle: Genetic Drift

Genetic drift is the random change in allele frequencies from one generation to the next due to chance alone (sampling error).
Its effect is significant only in small populations, where only a few individuals breed. In a small, isolated population the members interbreed among themselves, and by chance some alleles may become very common or may be lost.
Examples are the founder effect and the bottleneck effect.
In a large population, chance fluctuations average out, so drift is negligible. High variability or a constant low mutation rate do not cause drift.
So random genetic drift probably results from interbreeding within this (small) population.

Genetic drift is the random change in allele frequencies due to chance (sampling error) from one generation to the next.
In a large population, chance effects average out. In a small, isolated population, only a few individuals breed, so chance can greatly change allele frequencies; some alleles may be lost or fixed.
Examples are the founder effect and the bottleneck effect.
So genetic drift operates in a small isolated population.

In an isolated population there is no gene flow (no migration in or out).
In a small isolated population, allele frequencies can change by chance alone from one generation to the next. This random change is called genetic drift (e.g. founder effect).
Mutation and natural selection can also change allele frequencies, but the change characteristic of small isolated populations is drift.
So, as per the answer key, allele frequency in an isolated population may change due to genetic drift.