Which of the following represents a vector ?

Options

A. `leftrightarrow`
B. `rarr`
C. `underset(A)larr`
D. `underset(A)rarr`

Explanation

To determine which of the given options represents a vector, we first need to understand what a vector is. A vector is a physical quantity that has both magnitude and direction.

### Step-by-Step Solution:

1. **Understanding Vectors**:
- A vector is defined by two key components: magnitude (how much) and direction (which way). Examples of vectors include displacement, velocity, acceleration, and force.

2. **Identifying Notation**:
- Vectors are typically represented in physics using a bold letter or with an arrow above the letter. For instance, velocity can be represented as **v** or \( \vec{v} \).

3. **Evaluating Options**:
- We need to look at the provided options and analyze each one to see if it meets the criteria of having both magnitude and direction.

4. **Analyzing Each Option**:
- **Option 1**: If it is just a single letter without any indication of direction or magnitude, it cannot represent a vector.
- **Option 2**: If it is just a number or magnitude without a direction, it cannot represent a vector.
- **Option 3**: If it is a directional term without a magnitude, it cannot represent a vector.
- **Option 4**: If it is a symbol that includes both magnitude and direction (like \( \vec{v} \)), this option represents a vector.

5. **Conclusion**:
- After analyzing all options, we conclude that only the option that includes both magnitude and direction (option 4) correctly represents a vector.

### Final Answer:
- The correct option that represents a vector is **Option 4**.