What is the name of the sac at the head of a sperm that contains enzymes for fertilization?

The sac at the head of a sperm that contains enzymes crucial for fertilization is known as the acrosome. This specialized structure is essential for the sperm's ability to penetrate the outer layers of the egg (oocyte) during the fertilization process. The enzymes contained within the acrosome, such as hyaluronidase, help to break down the protective zona pellucida that surrounds the egg, allowing the sperm to reach the plasma membrane of the oocyte and facilitate fusion.

In contrast, the axoneme refers to the central strand of a cilium or flagellum that is involved in the motility of the sperm rather than fertilization. A follicle is a structure in the ovary that contains developing oocytes and is not related to the sperm's fertilization capabilities. The oocyte is the female gamete itself, not a structure found in the sperm. Thus, the acrosome serves a specific and vital role in the fertilization process, making it the correct answer.