Final answer:
Sperm penetrate the corona radiata through the acrosomal reaction, which involves the release of enzymes that break down the protective layers surrounding the oocyte, eventually leading to a single sperm fusing with the oocyte's plasma membrane.
Step-by-step explanation:
The factor that allows sperm to penetrate the corona radiata is an event known as the acrosomal reaction. This reaction involves the release of digestive enzymes from the acrosome, which is a cap-like structure on the sperm's head. These enzymes degrade the extracellular matrix of the corona radiata, which is a layer of granulosa cells that surrounds the oocyte. Multiple sperm undergo the acrosomal reaction, contributing to the weakening of the corona radiata and the zona pellucida, another layer that must be penetrated for fertilization to occur. Ultimately, a single sperm's plasma membrane fuses with that of the oocyte, allowing it to release its genetic material into the oocyte.