To penetrate the zona pellucida of the ovum, the sperm

(a) passes down a channel in the protective layer to reach the egg membrane
(b) stimulates opening of ion channels along the egg membrane by releasing sodium ions from the acrosomal cap
(c) releases a hormone that signals the egg to dissolve the thick coating
(d) releases digestive enzymes from the acrosomal cap
(e) uses the swimming action of the flagellum to move inside the egg

(D) releases digestive enzymes from the acrosomal cap.

Explanation:

The zona pellucida is an outer layer which covers the ovum from outside. It is composed from the extra-cellular matrix of glycoproteins. The sperm head binds to the ZP3 glycoprotein of zona pellucida. This binding process triggers the acrosome cap of the sperm to burst. The acrosomal cap releases the enzymes, which helps the sperm to penetrate the zona pellucida layer and cause fertilization of ovum.    

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