The correct option is C Cline
The diverse climate and the seasonal variations lead to the formation of several zones having flora and fauna with the phenotypic difference. An ecocline or simply cline describes an ecotone in which a series of biocommunities display a continuous gradient. The term was coined by the Huxley in 1938. Clines are the source of genetic and phenotypic variations. It consists of different forms of species, that exhibit gradual phenotypic and / or genetic differences over a geographical area, typically as a result of diverse environmental condition.