Phosphate ester of X is called a nucleotide. X is

Options

A. adenine
B. guanine
C. nucleoside
D. thymine

Answer

To solve the question "Phosphate ester of X is called a nucleotide. X is?", we need to understand the components of a nucleotide and identify what X represents.

### Step-by-Step Solution:

1. **Understanding Nucleotide Composition**:
- A nucleotide is composed of three main components:
- A nucleobase (which can be adenine, guanine, cytosine, or thymine).
- A five-carbon sugar (which can be ribose or deoxyribose).
- One or more phosphate groups.

2. **Identifying the Role of X**:
- The question states that the "phosphate ester of X is called a nucleotide."
- This implies that X must be a component that, when combined with phosphate, forms a nucleotide.

3. **Defining Nucleoside**:
- A nucleoside consists of:
- A nucleobase (adenine, guanine, cytosine, or thymine).
- A five-carbon sugar (ribose or deoxyribose).
- Importantly, a nucleoside does not contain phosphate groups.

4. **Connecting Nucleoside to Nucleotide**:
- A nucleotide can be defined as a nucleoside plus one or more phosphate groups.
- Therefore, if X is a nucleoside, then the phosphate ester of X (which is a nucleoside) would indeed be a nucleotide.

5. **Conclusion**:
- Since X must be a nucleoside for the phosphate ester of X to be classified as a nucleotide, we conclude that:
- **X is a nucleoside**.

### Final Answer:
X is a nucleoside.