Which of the following accurately describes the activity of proto-oncogenes?

Proto-oncogenes are normal genes that play a crucial role in regulating cell growth and differentiation. They are involved in promoting cell division when necessary. However, when these genes undergo mutations or become dysregulated, they can transform into oncogenes, which can lead to uncontrolled cell proliferation and contribute to cancer development. This transition emphasizes the potential of proto-oncogenes to promote cancer under certain conditions, making option B the correct choice.

In contrast, the other statements do not accurately capture the role of proto-oncogenes. The assertion that they inhibit cell division is contrary to their function; they actively promote growth. The claim that they are always active is misleading, as proto-oncogenes can be expressed in a regulated manner depending on cellular needs. Lastly, while DNA repair is critical for maintaining genomic integrity, proto-oncogenes are not primarily involved in DNA repair processes. Therefore, the understanding of proto-oncogenes as genes that can lead to cancer when mutated is key to grasping their function in cellular biology.