Assertion : External parasitism is generally marked by much more exterme specialisation than internal parasitism.
Reason : The structure of an internal parasite is usually very complex possessing suckers, reproductive organs, etc.

Answer

D

Explanation

Internal parasitism is generally marked by much more extreme specialisation than external parasitism, as shown by many protists and invertebrate parasites that infect humans. The more closely the life of the parasite is linked with that of its host, the more its morphology and behaviour are likely to have been modified during the course of its evolution. Conditions within the body of an organism are different from those encountered outside and are apt to be much more constant. Consequently, the structure of an internal parasite is often simplified and unnecessary structure are lost as it evolves.