The complicated molecules that make up living things usually contain carbon. why is carbon so important in these molecules

A. Carbon can form four strong covalent bonds.

Explanation:

Carbon can form up to 4 covalent bonds, and it can form reasonably strong bonds with nitrogen, oxygen, and other heteroatoms.

Carbon also has an almost unique ability to catenate: to form long chains with

C − C linkages, and along the chain there can be regions of different reactivity (by the presence of functional groups).

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.