The correct option is D Ensure fusion of gamete of same species.
A fertilizin is a proteoglycan secreted from the cortical region of the egg cytoplasm, that interacts with the antifertilizin of the sperm of the same species. This makes the sperms stick to the surface of the egg. An antifertilizin is a protein present in the plasma membrane of the sperm. Fertilizin acts as a receptor for antifertilizin and the reaction between fertilizin and antifertilizin makes the sperm more capable of fertilizing the egg of the same species.