Impala share the african plains with other grazing animals and predators. which change would lead to a decrease in the impala population?

So, the Impala share their food with other grazing animals. These animals are their competition.

If there is an increase in the competition, that is in the number of other grazing animals, such as other species of antelope (Impala are antelopes)  then the population of impala would decrease in size.

So the answer is:  an increase in the number of other grazing animals,