Preparation of sperm before penetration of ovum:

The correct answer is option 'D' - capacitation.

Explanation:

Sperm cells undergo a series of changes and preparations in order to be able to fertilize the ovum. This process is known as capacitation. Let's discuss the steps involved in the preparation of sperm before penetration of the ovum in detail:

1. Spermatogenesis:
- Spermatogenesis is the process of sperm cell production in the testes.
- It involves the differentiation of germ cells into mature sperm cells through a series of mitotic and meiotic divisions.
- Spermatogenesis takes place in the seminiferous tubules of the testes.

2. Spermiogenesis:
- Spermiogenesis refers to the maturation of the spermatids into fully functional sperm cells.
- During this process, the round spermatids undergo extensive morphological changes to acquire the characteristic shape of sperm cells.
- The spermatids develop a head, midpiece, and a tail, which collectively enable them to swim towards the ovum.

3. Capacitation:
- Capacitation is the final step in the preparation of sperm before they can penetrate the ovum.
- It occurs within the female reproductive tract, specifically in the fallopian tubes.
- Capacitation involves the removal of glycoprotein and cholesterol molecules from the surface of the sperm cell.
- This removal allows the sperm cell to become more flexible and acquire enhanced motility.
- Capacitation also leads to changes in the sperm cell membrane, allowing it to bind to the zona pellucida, a glycoprotein layer surrounding the ovum.

4. Spermiation:
- Spermiation is not related to the preparation of sperm before penetration of the ovum.
- It refers to the release of mature sperm cells from the seminiferous tubules into the lumen of the testes.

5. Cortical reaction:
- The cortical reaction occurs after fertilization, not before the penetration of the ovum.
- It is a process in which cortical granules present in the cytoplasm of the ovum are released into the perivitelline space.
- The released enzymes from cortical granules prevent polyspermy, which is the entry of multiple sperm cells into the ovum.

In conclusion, the correct answer is option 'D' - capacitation, as it is the final step in the preparation of sperm before they can penetrate the ovum.