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A sperm cell consists of three main parts: the head, the midpiece, and the tail. The head contains genetic material (DNA) and is covered by a cap called the acrosome, which contains enzymes that help the sperm penetrate the egg. The midpiece contains numerous mitochondria, which provide energy for the sperm, and the tail (flagellum) helps the sperm move towards the egg.

(a) Nucleus only: This option suggests that the head of sperm only contains the nucleus, which is not entirely accurate as it also contains the acrosome. (b) Nucleus and mitochondria: This option suggests that the head of sperm contains both the nucleus and mitochondria. However, the mitochondria are primarily located in the midpiece of the sperm. (c) Mitochondria and acrosome: This option suggests that the head of sperm contains mitochondria and acrosome, but, as mentioned earlier, the mitochondria are mainly in the midpiece. (d) Nucleus and acrosome: This option correctly identifies the two primary components of the sperm head, which are the nucleus containing the genetic material and the acrosome containing enzymes.

Based on the analysis of each answer choice, the correct answer is (d) Nucleus and acrosome, as the head of sperm contains both the nucleus with the genetic material and the acrosome that assists in penetrating the egg with the help of enzymes it contains.