What type of virus contains reverse transcriptase?

Reverse transcriptase is an enzyme that is crucial for the replication of retroviruses. Retroviruses are a specific group of RNA viruses that have the unique ability to convert their RNA genome into DNA after entering a host cell. This process is fundamental to their lifecycle, as it allows the viral DNA to integrate into the host's genome, enabling the virus to hijack the cellular machinery for its replication and propagation.

The presence of reverse transcriptase sets retroviruses apart from other types of viruses. While some RNA viruses may have RNA polymerases to replicate their RNA, it is the retroviruses specifically that utilize reverse transcriptase to synthesize DNA from their RNA template. Proviruses refer to the integrated form of a retrovirus' genetic material within a host cell's DNA, rather than a standalone virus type. Therefore, the key characteristic of retroviruses is their reliance on reverse transcriptase, making them the correct choice for this question.