The number of chromosomes is reduced to half during

Options

A. mitosis
B. meiosis II
C. Meiosis I
D. fertilisation.

Explanation

To solve the question regarding the reduction of chromosome number to half, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Processes**: We need to analyze the four processes mentioned: mitosis, meiosis I, meiosis II, and fertilization.

2. **Mitosis**: In mitosis, a single cell divides to produce two daughter cells. Importantly, the chromosome number remains the same as the parent cell. Therefore, mitosis does not reduce the chromosome number.

3. **Meiosis II**: In meiosis II, the cells involved are already haploid (having half the number of chromosomes). The process involves the division of these haploid cells into two more haploid cells. Thus, there is no further reduction in chromosome number during meiosis II.

4. **Meiosis I**: In meiosis I, the parent cell is diploid (having two sets of chromosomes). During this process, the cell divides to produce two daughter cells that are haploid. This is the key step where the chromosome number is reduced to half, making meiosis I known as a reductional division.

5. **Fertilization**: Fertilization involves the fusion of two gametes (each haploid) to form a diploid zygote. This process actually doubles the chromosome number rather than reducing it.

6. **Conclusion**: The only process among the options provided that results in the reduction of chromosome number to half is meiosis I.

### Final Answer:
The number of chromosomes is reduced to half during **meiosis I**.

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