Final answer:
The correct statement is that a tumor virus must become stably integrated into the host genome. Tumor viruses can lead to cancer by affecting proto-oncogenes or carrying oncogenes, and they are found in various species and cell types.
Step-by-step explanation:
Regarding tumor viruses and oncogenes, the statement that is correct is C) a tumor virus must become stably integrated into the host genome. Tumor viruses can lead to cancer by either carrying an overactive oncogene themselves or by inserting their genome near a proto-oncogene in the host genome, thus causing overexpression and uncontrolled cellular proliferation.
This is particularly true for retroviruses, where viral genome integration is a requisite part of their lifecycle. Contrary to some options provided, tumor viruses have been observed in a variety of species, including rodents and primates, not just avians.
Also, tumor viruses do not solely encode enzymes for DNA replication nor only transform epithelial cells; they can infect various cell types and often hijack the host machinery for replication or use their own enzymes, such as reverse transcriptase in the case of retroviruses.