**Core Concept**
During spermatogenesis, the process of forming sperm cells in the male reproductive system, the number of chromosomes undergoes a significant reduction to ensure that the resulting sperm cells have the correct number of chromosomes. This reduction is crucial for proper fertilization and the development of a healthy embryo.

**Why the Correct Answer is Right**
The reduction in chromosome number occurs during meiosis, specifically during the first meiotic division (meiosis I). During this process, homologous chromosomes, which are pairs of chromosomes that carry the same genes, separate from each other. This separation is made possible by the breaking of the synaptonemal complex, a protein structure that holds the homologous chromosomes together. As a result, each resulting cell receives only one chromosome from each pair, effectively reducing the number of chromosomes from diploid (46) to haploid (23).

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because the correct stage of meiosis where chromosome number reduction occurs is meiosis I, not meiosis II.
**Option B:** This is not a valid option.
**Option D:** This is incorrect because the correct stage of meiosis where chromosome number reduction occurs is meiosis I, not the beginning of spermatogenesis.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the reduction in chromosome number during spermatogenesis ensures that each sperm cell has only 23 chromosomes, which is the correct number for fertilization and the development of a healthy embryo.

**Correct Answer:** C. Meiosis I.