Assertion: The difference of two vectors A and B can be treated as the sum of two vectors.
Subtraction of vectors can be defined in terms of addition of vectors.

Explanation

To solve the question regarding the assertion and reason about vector subtraction and addition, we can break down the solution into the following steps:

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that the difference of two vectors A and B can be treated as the sum of two vectors. Mathematically, this can be expressed as:
\[ A - B = A + (-B) \]
- Here, \(-B\) is the vector that has the same magnitude as B but points in the opposite direction.

2. **Understanding the Reason**:
- The reason provided states that subtraction of vectors can be defined in terms of addition of vectors. This is indeed true as shown in the previous step.
- The operation of subtracting vector B from vector A can be rephrased as adding the negative of vector B to vector A.

3. **Evaluating the Truth of the Assertion and Reason**:
- Both the assertion and the reason are correct statements in vector algebra.
- The assertion is true because we can express the difference of two vectors as the sum of one vector and the negative of another.
- The reason is also true as it correctly explains how vector subtraction can be defined using vector addition.

4. **Conclusion**:
- Since both the assertion and reason are true, and the reason explains the assertion, we conclude that the correct option is:
- **Option A**: Both assertion and reason are true, and the reason is the correct explanation of the assertion.