After a sperm has penetrated an ovum in the process of fertilization, entry of further sperms is prevented by

The correct option is "formation of fertilization membrane".

Here is a step-by-step biological explanation of how this process works during fertilization:

1. Initial Contact and Depolarization (Fast Block to Polyspermy):
When a sperm makes contact with the plasma membrane of the ovum (egg), it triggers an immediate wave of depolarization across the egg membrane. This change in membrane potential acts as a temporary barrier (fast block) to prevent other sperms from fusing with the egg.

2. Cortical Reaction (Slow Block to Polyspermy):
The binding of the sperm also stimulates the release of calcium ions within the egg. This increase in intracellular calcium triggers the cortical reaction, wherein cortical granules located just beneath the egg's plasma membrane fuse with the membrane and release their enzymatic contents into the perivitelline space (the space between the plasma membrane and the vitelline membrane).

3. Formation of the Fertilization Membrane:
These cortical enzymes modify the vitelline membrane and detach it from the plasma membrane. The vitelline membrane then hardens, thickens, and transforms into a robust physical barrier known as the fertilization membrane. This membrane completely seals the egg, preventing any additional sperm (polyspermy) from entering the ovum and ensuring that only a single sperm fertilizes the egg.