Step 1: Understanding capsids.
A capsid is the protein shell that surrounds the genetic material of a virus. It is composed of protein subunits called capsomers.
Step 2: Analyzing the options.
(A) proteins: Correct — Capsids are made up of protein subunits that form a protective layer around the viral genome.
(B) nucleic acids: Incorrect — Nucleic acids make up the genome of the virus, but not the capsid.
(C) lipids: Incorrect — Lipids are found in the viral envelope, but not in the capsid itself.
(D) sterols: Incorrect — Sterols are a type of lipid, but they do not make up the viral capsid.
Step 3: Conclusion.
The correct answer is (A) proteins.