The zona pellucida is a layer of glycoproteins that surrounds the mammalian oocyte. It plays a crucial role in fertilization by allowing only one sperm to penetrate and fuse with the oocyte. The receptors for sperm binding are present on the zona pellucida.

Explanation:

• The mammalian oocyte is surrounded by three layers - the corona radiata, the zona pellucida, and the vitelline membrane.

• The corona radiata is a layer of granulosa cells that surround the oocyte and is shed during ovulation.

• The zona pellucida is a thick layer of glycoproteins that surrounds the oocyte and is responsible for the species-specific recognition and binding of sperm.

• The vitelline membrane is a thin layer that lies beneath the zona pellucida and surrounds the oocyte plasma membrane.

• Sperm must first penetrate the corona radiata and then bind to the zona pellucida before they can fuse with the oocyte plasma membrane and fertilize the egg.

• The zona pellucida contains several glycoproteins, including ZP1, ZP2, and ZP3, which are responsible for binding to sperm.

• Sperm bind to the zona pellucida via their surface receptors, which recognize and bind to the ZP3 glycoprotein.

• The binding of sperm to the zona pellucida triggers a series of events that lead to the acrosome reaction, in which the sperm releases enzymes that allow it to penetrate the zona pellucida and fuse with the oocyte plasma membrane.

• After fertilization, the zona pellucida hardens to prevent polyspermy, or the fertilization of the egg by more than one sperm.

Therefore, the correct answer is option B, the zona pellucida, as it is the layer of the oocyte that contains the receptors for sperm binding in mammals.

B