Which one of the following interspecies relationships has a negative effect on the abundance of both species?
a. commensalism
b. parasitism
c. predation
d. competition
(i know that b is wrong)

D. Competition. Competition is a negative type of interaction among organism of the same (intraspecific) and different (interspecific) species. Organisms often compete when there is less resources in the environment. Because other organisms may be consuming or defending a resource, other organisms that need it need to compete, thus reducing the growth and survival of both competing organisms.

When plants die fungi breaks them down