Which enzyme do retroviruses such as HIV carry?

Retroviruses, such as HIV, are unique among viruses due to their ability to reverse transcribe their RNA genome into DNA after entering a host cell. This process is facilitated by the enzyme called reverse transcriptase, which is essential for the viral replication cycle.

Reverse transcriptase synthesizes complementary DNA (cDNA) from the viral RNA template, which is then integrated into the host's genome by another viral enzyme called integrase. This integration allows the virus to hijack the host's cellular machinery to produce new viral particles.

The other options do not serve the same function within retroviruses. DNA polymerase, for instance, is involved in DNA replication but is not the enzyme characteristic of retroviruses. Helicase is important for unwinding DNA or RNA structures but does not play a role in the specific replication mechanism of retroviruses. Ribonuclease refers to enzymes that break down RNA; while these may have roles in other contexts, they are not key to the retroviral replication process. Thus, reverse transcriptase is the critical enzyme carried by retroviruses like HIV for their successful replication and life cycle.