In enzyme code , one of the following is always in a single digit

Options

A. class
B. subclass
C. sub sub class
D. serial number

Answer

To solve the question, "In enzyme code, one of the following is always in a single digit," we will analyze the enzyme code structure and the options provided.

### Step-by-Step Solution:

1. **Understanding Enzyme Code**: The enzyme code is a unique four-digit code assigned to each enzyme based on the universal nomenclature. This code consists of four segments: class, subclass, sub-subclass, and serial number.

2. **Identifying the Segments**:
- **Class**: This is the first digit of the enzyme code.
- **Subclass**: This is the second digit.
- **Sub-subclass**: This is the third digit.
- **Serial Number**: This is the fourth digit.

3. **Analyzing the Classes**: According to the universal nomenclature, there are only seven classes of enzymes:
- Oxidoreductases
- Transferases
- Hydrolases
- Lyases
- Isomerases
- Ligases
- Translocases

4. **Determining the Digit Requirement**:
- Since there are only seven classes, the first digit (class) can only be a single digit (1 to 7).
- The subclass, sub-subclass, and serial number can vary and may consist of more than one digit due to the vast number of enzymes.

5. **Conclusion**: The only segment that is always represented by a single digit is the **class**. Therefore, the answer to the question is that the **class** is always in a single digit.

### Final Answer:
The answer is **class**.