[A] : Sponges are primitive multicellular animals that probably evolved a multicellular structure independently of other animals .
[R] : Sponges are stationary animals and remain fixed to substratum while water passes over them .

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 regarding sponges, 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 "Sponges are primitive multicellular animals that probably evolved a multicellular structure independently of other animals."
- This statement is true. Sponges belong to the phylum Porifera, which is indeed considered the simplest and most primitive form of multicellular animals. They have a unique cellular organization that is distinct from other animal phyla.

2. **Understanding the Reason (R)**:
- The reason states that "Sponges are stationary animals and remain fixed to substratum while water passes over them."
- This statement is also true. Sponges are sessile organisms, meaning they do not move from one place to another. They are attached to a solid surface, such as rocks or reefs, and water flows through their bodies, allowing them to filter feed.

3. **Evaluating the Relationship Between A and R**:
- While both statements are true, we need to determine if the reason (R) correctly explains the assertion (A).
- The assertion discusses the evolutionary aspect of sponges being primitive multicellular animals, while the reason describes their stationary nature. The reason does not explain why sponges evolved as primitive multicellular animals; it merely describes their behavior and habitat.

4. **Conclusion**:
- Since both the assertion and reason are true, but the reason does not provide a correct explanation for the assertion, the correct option is:
- **Both A and R are true, but R is not the correct explanation of A.**

### Final Answer:
The correct answer is the second option: "Both A and R are true, but R is not the correct explanation of A."