The study of structures of organisms, their cells and their organelles at molecular level is known as

Answer: Option A

Molecular biology is the branch of biology that deals with the study of the structures of organisms, their cells, and their organelles at the molecular level.

It focuses on understanding the interactions between various systems of a cell, including the interactions between DNA, RNA, and protein synthesis.

This field combines aspects of genetics, biochemistry, and cell biology to explain how biological molecules give rise to the processes that occur within living cells.

Therefore, the correct term for this study is Molecular biology.