Assertion : A bony fish can stay at a particular depth without expending energy is a swimming contrary to the cartilaginous fishes.
Reason : In bony fishes, the swim bladder is present which helps in buoyancy.
(a) If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
(b) If both Assertion and Reason are true ,but the Reason is not the correct explanation of the Assertion.
(c) If Assertion is true, but Reason is false
(d) If both Assertion and Reason are false.

Options

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

Answer

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "a bony fish can stay at a particular depth without expending energy in swimming, contrary to cartilaginous fish."
- This is true because bony fishes, such as those in the class Osteichthyes, have adaptations that allow them to maintain their position in the water column without continuous swimming.

2. **Understanding the Reason**:
- The reason states that "in bony fishes, the swim bladder is present which helps in buoyancy."
- This is also true. The swim bladder is a gas-filled organ that allows bony fishes to control their buoyancy, enabling them to maintain depth without using energy to swim.

3. **Evaluating the Relationship**:
- The reason provided explains why the assertion is true. The presence of the swim bladder directly contributes to the ability of bony fishes to stay at a certain depth without expending energy, which contrasts with cartilaginous fishes that lack this adaptation.

4. **Conclusion**:
- Since both the assertion and reason are true, and the reason correctly explains the assertion, the correct answer is (a): If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

### Final Answer:
**(a) If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.**