Answer: A. they are overcrowded
A limiting factor is the factor which affects the growth, abundance and distribution of population of species in the living environment. It is of two types: density dependent factor and density independent factor. The density dependent limiting factor results in the change of the per capita population of the living species, with the increase in the density of the population of species. Density independent limiting factor affects per capita population of the living species, irrespective of the density of the population.
Parasitism is a phenomena in which one organism feeds over the body of other to meet it's food, energy requirements and to grow in numbers. Parasitism is a density dependent limiting factor because more denser the population more profound will be the effect of parasitism. As, parasites will get more number of hosts to acquire their food, and the infection will be transferred easily among large number of individuals in the population.
Hence, overcrowded population of host organisms is most likely to be parasitized.
