The phase of cell division by which the chromosome number is halved is known as:

Options

A. mitosis
B. meiosis I
C. meiosis II
D. karyokinesis

Answer

To answer the question regarding the phase of cell division by which the chromosome number is halved, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Types of Cell Division**:
- There are two main types of cell division: mitosis and meiosis.
- Mitosis is typically referred to as equational division, meaning the daughter cells have the same chromosome number as the parent cell.

2. **Identify the Purpose of Mitosis**:
- Mitosis results in two daughter cells that are genetically identical to the parent cell. For example, if the parent cell has a diploid number of chromosomes (2N), the daughter cells will also have 2N.

3. **Understand Meiosis**:
- Meiosis is a specialized type of cell division that occurs in the formation of gametes (sperm and eggs).
- It consists of two successive divisions: meiosis I and meiosis II.

4. **Focus on Meiosis I**:
- Meiosis I is known as reductional division because it reduces the chromosome number by half.
- For instance, if the parent cell has a diploid number (2N), after meiosis I, the resulting cells will have a haploid number (N).

5. **Conclusion**:
- Since the question specifically asks for the phase of cell division where the chromosome number is halved, the correct answer is meiosis I.

### Final Answer:
The phase of cell division by which the chromosome number is halved is known as **meiosis I**.

---