Answer:
A. genetic drift (bottleneck).
Explanation:
Genetic drift can cause big losses of genetic variation for small populations. In our case, we already had a small population so killing further reduced the genetic variation in the population.
Disruptive selection is not correct because the plants died randomly (particular phenotype were not favored). Natural selection and stabilizing selection are not correct for same reason.
Gene flow is incorrect because we did not have any individuals coming in into the population!