Chromosome number is reduced during meiosis because the process consists of :-

Answer

**Step-by-Step Solution:**

1. **Understanding Meiosis**: Meiosis is a type of cell division that reduces the chromosome number by half, resulting in the formation of four haploid cells from one diploid cell. It consists of two successive divisions: meiosis I and meiosis II.

2. **Meiosis I - Reductional Division**: The first division, meiosis I, is known as the reductional division. During this phase, homologous chromosomes are separated, leading to a reduction in chromosome number. Each daughter cell receives one chromosome from each homologous pair, resulting in two haploid cells.

3. **Meiosis II - Equational Division**: The second division, meiosis II, is similar to mitosis and is referred to as the equational division. Here, the sister chromatids of each chromosome are separated, but the chromosome number remains haploid.

4. **Chromosome Replication**: Prior to meiosis, during the S phase of interphase, DNA replication occurs once. This means that each chromosome consists of two sister chromatids at the start of meiosis I. However, there is no further replication of chromosomes between meiosis I and meiosis II.

5. **Conclusion**: Therefore, the chromosome number is reduced during meiosis because the process consists of two cell divisions (meiosis I and meiosis II) and only a single round of chromosome replication that occurs before meiosis begins.

**Final Answer**: The correct option is "two cell divisions and only a single round of chromosome replication."

---