The correct answer is Option (2) - Zona pellucida.

The structure that blocks the entry of additional sperm into the egg is the "Zona pellucida."

The zona pellucida is an extracellular matrix that surrounds the egg cell (oocyte). It plays a crucial role during fertilization by preventing polyspermy, which is the entry of multiple sperm into a single egg. After the first sperm penetrates the zona pellucida and fuses with the egg, the zona becomes "hardened," preventing other sperm from entering and ensuring that only one sperm fertilizes the egg. This helps maintain the integrity of the genetic material and ensures the development of a viable embryo.