What is the reason carbon atoms are the most versatile building blocks of the molecules used by living organisms?

Atomic number of carbon is 6. So, 4 valence electrons are present.

Therefore, it can form 4 covalent bonds with varying bond angles by sharing its valence electrons.

Catenation is also an important property of carbon. Catenation is bonding with atoms of same element. Carbon skeleton can be formed in any direction and can vary in length, branching, and ring structure.

Elements required for making most of the molecules in living organisms are:

C, H, N, O, P and S

Carbon easily form covalents with other 5 elements.

These properties make carbon most versatile building blocks of the molecules used by living organisms.