Question

which of the following is not a main class of organic compounds?
a water
b proteins
c nucleic acids
d lipids

Explanation:

Organic compounds are carbon - based and include proteins, nucleic acids, and lipids. Water ($H_2O$) has no carbon and is an inorganic compound, so it's not a main class of organic compounds. Proteins, nucleic acids, and lipids are all main classes of organic compounds.

Answer:

a. water