What is competition a cause of?
a. natural competition
b. natural symbioses
c. natural capacity
d. natural selection

The answer is d.
natural selection.
populations of animals are controlled by many factors. 
natural selection  is a broad term that describes one effect of these controls on population. for example, one form of population control that can result in natural selection is competition. there are a number of essential resources upon which animals' lives depend. whenever these resources are limited, animals are forced to compete for survival. competition can be  intraspecific  , between the same species, or interspecific, between different species. since resources are rarely abundant in any given environment, competition can be fierce. three resources that animals are likely to complete for are space, water, and food.