Ans. A. The acrosome reactionThe acrosome reaction causes a fusion of the plasma membrane and the acrosomal membrane of the sperm, with subsequent release of proteolytic enzymes that help the sperm enter the ovum. The zona reaction and Pro nuclei formation occur after the sperm has entered the ovum. Sperm enter the perivitelline space after penetration; there is no evidence that this space has any role in penetration. Cumulus expansion assists in movement of the sperm through the mass of granulosa cells for the sperm to get to the surface of the zona pellucida. However, the cumulus cells do not assist in actual penetration of the zona.