Answer- A. Cell cycle will stop at GI phaseRb is a tumor suppressor gene. It normally arrests cell division at G1-S phase. Phosphorylation of Rb gene allows the cell to divide, hence inhibition of phosphorylation (which is the constitutive scenario for Rb gene) arrests the cell in GI phase.RB (Retinoblastoma) geneLocated on chromosome on 13q14Tumor suppressive pocket protein that binds E2F transcription factors in hypophophorylated stateKey negative regulator of G1/S cell cycle transition . Tumors associated: Retinoblastoma, osteosarcoma, Glioblastoma, small cell carcinoma of lung, CA breast & CA bladderSequence: Go- G1- S- G2 - M