Which mechanism is most likely to control populations of species in a diverse ecosystem? 

A)  predators 
B)  disease organisms 
C)  pesticides 
D)  parasites

Predators play a crucial role in controlling populations of species in diverse ecosystems by preying on the individuals, thus reducing their numbers. This helps in maintaining a balance in the ecosystem and prevents any one species from dominating the others. Disease organisms, pesticides, and parasites may also play a role in controlling populations, but they are not as effective or widespread as predators. Additionally, the use of pesticides can have unintended consequences on other non-target species in the ecosystem.