Which of the following would be most likely to decrease genetic variability? question 11 answers bottleneck genetic drift mutation random mating

Mutation and random mating would increase, not decrease genetic variability - especially mutation, as it is the thing that introduces genetic variability into species. Genetic drift is a change in frequency of an allele, so it changes th variability, but does not decrease it necessarily, can also increase it.

the correct answer is bottleneck: it decreases the variability to only the genes that "fitted in the bottleneck".