The correct option is
A Study of physico-chemical nature of biomolecules
Molecular biology is a branch of biology that concerns the molecular basis of biological activity between biomolecules in the various systems of a cell, including the interactions between DNA, RNA, proteins and their biosynthesis, as well as the regulation of these interactions. Hence, molecular biology is the study of the physicochemical nature of biomolecules.
So, the correct answer is 'study of the physico-chemical nature of biomolecules'.