Final answer:
Rb activity is regulated by phosphorylation, and it works to repress cell cycle progression by inhibiting the transcription of genes needed for the G1/S transition.
Step-by-step explanation:
Rb (retinoblastoma protein) activity is regulated by phosphorylation; it represses cell cycle progression by preventing the transcription of genes necessary for the G1/S transition. In its active, unphosphorylated state, Rb binds to E2F transcription factors, blocking the production of these proteins. As the cell grows, Rb becomes phosphorylated, leading to its inactivation and the release of E2F. This release allows E2F to activate genes needed for the cell to progress through the G1/S checkpoint. Therefore, answer (A) Regulated by phosphorylation; Represses cell cycle progression is correct.