The answer is C. Physical structure.
Different species can share dietary habits, ecological niches, and reproductive method although they are not related. So, classifying organisms based on these features is not helpful a lot. It is known that different species differ in DNA sequence. Since physical structures are determined by DNA sequences, physical structures could show in first sight difference between species. Therefore, to classify organism found in the field, the scientist will start from comparing its physical structures to the other known organisms.
