Which of the following is not a primidine base ?

Options

A. Uracil
B. Guanine
C. Cytosine
D. Thymine

Explanation

To determine which of the following is not a pyrimidine base, we first need to understand the classification of nitrogenous bases in nucleic acids.

### Step-by-Step Solution:

1. **Identify the Categories of Nitrogenous Bases**:
- Nitrogenous bases are classified into two main categories: pyrimidines and purines.
- Pyrimidines include:
- Cytosine (C)
- Thymine (T)
- Uracil (U)
- Purines include:
- Adenine (A)
- Guanine (G)

2. **List the Given Options**:
- Let's assume the options provided are:
- A) Cytosine
- B) Thymine
- C) Uracil
- D) Guanine

3. **Analyze Each Option**:
- **Cytosine (C)**: This is a pyrimidine base.
- **Thymine (T)**: This is also a pyrimidine base.
- **Uracil (U)**: This is a pyrimidine base found in RNA.
- **Guanine (G)**: This is a purine base.

4. **Identify the Non-Pyrimidine Base**:
- From the analysis, we see that Guanine (G) is the only base that is not a pyrimidine; it is a purine.

5. **Conclusion**:
- Therefore, the answer to the question "Which of the following is not a pyrimidine base?" is **Guanine (D)**.

### Final Answer:
**D) Guanine**