Which component is present in some viruses but absent in others?
a.)nucleic acid
b.)capsid
c.)envelope
d.)cell membrane

Ans. Option C). envelope.

Viruses are small infectious particles that represent connecting link between non-living and living things. They are made up of nucleic acid (either RNA or DNA) and a protective coat or capsid, which is composed of protein molecules. Viruses lack cell membranes.

In some virus, an another layer is also present outside the capsid, known as envelope. These viruses are known as enveloped viruses. On the other hand, some viruses lack envelopes and are known as naked viruses.

Thus, the correct answer is option C). 'envelope.'