A group of organisms in a well defined geographical area,share or compete for similar resources,potentially interbreed are known as :-

$\begin{array}{1 1}(a)\;\text{An ecotype}\\(b)\;\text{Ecological family}\\(c)\;\text{Population}\\(d)\;\text{Phylum}\end{array}$

A group of organisms in a well defined geographical area,share or compete for similar resources,potentially interbreed are known as population.
Hence (c) is the correct answer.