[A]: Protostele is most simple primitive type of stele.
[R]: According to Histogen theory there are three distinct apical layers which give rise to distinct tissue system of the body.

Options

A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false

Answer

To solve the question, we need to analyze both the assertion (A) and the reason (R) provided.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that "Protostele is the most simple primitive type of stele."
- A protostele is indeed a type of vascular tissue arrangement found in some primitive vascular plants. It consists of a central core of xylem surrounded by phloem, and it does not have any pith. This statement is true.

2. **Understanding the Reason (R)**:
- The reason states that "According to Histogen theory, there are three distinct apical layers which give rise to distinct tissue systems of the body."
- The Histogen theory, proposed by Hanstein, describes the organization of the shoot apical meristem into three layers: dermatogen (outer layer), periblem (middle layer), and pleurome (inner layer). This statement is also true.

3. **Analyzing the Relationship Between A and R**:
- While both statements A and R are true, we need to determine if R is a correct explanation of A.
- The assertion about protostele does not directly relate to the histogen theory, which describes the organization of meristematic tissues. Therefore, R does not explain A.

4. **Conclusion**:
- Since both A and R are true but R is not the correct explanation of A, the correct option is: "If both A and R are true, but R is not the correct explanation of A."

### Final Answer:
The correct option is: **If both A and R are true, but R is not the correct explanation of A.**

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