What part of the hiv virus serves to protect the nucleic acid core?

The correct answer is -

Capsid (coat of protein).

Explanation:

All viruses (microscopic, acellular, non living organisms) form sheath of protein or a proteinaceous coat known as capsid, which surrounds the nucleic acid core (which may be DNA or RNA).

HIV (human immuno deficiency virus) has a RNA nucleic acid core that is single stranded and surrounded by a protein coat known as capsid.

Thus, capsid is the right answer.