Why was the reduction of the bison population to under 1,000 individuals a bottleneck event for the species? a. it drastically reduced the gene pool of the species. b. it reduced the territorial range available to the species. c. foreign genes were introduced by crossbreeding with cattle. d. it reduced the prevalence of recessive traits.

The best answer is that a) It drastically reduced the gene pool of the species - that's generally what bottleneck means.

b) The territorial range would be unchanged or, if anything, could grow (with reduced competition)

c) this would not be a bottleneck effect - or at least, this is not how we define it.
d) it reduced the number of genes for both dominant and recessive traits probably, so , not.