Assertion :- Companion cells are essential for functioning of sieve tubes.
Reason :- The companion cells help in maintaining the pressure gradient in sieve tubes.

Options

A. If both Assertion & Reason are True & Reason is a correct explanation of the Assertion.
B. If both Assertion & Reason are True but the Reason is a correct explanation of the Assertion.
C. If Assertion is True but the Reason is False.
D. If both Assertion & Reason are false.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "Companion cells are essential for the functioning of sieve tubes."
- Companion cells are specialized cells found in the phloem tissue of plants. They play a crucial role in supporting the sieve tube elements, which are responsible for the transport of nutrients.

2. **Understanding the Reason**: The reason given is "The companion cells help in maintaining the pressure gradient in sieve tubes."
- Companion cells are closely associated with sieve tubes and help in maintaining the pressure gradient necessary for the movement of sap through the sieve tubes. This pressure gradient is essential for the efficient transport of nutrients.

3. **Relationship between Companion Cells and Sieve Tubes**:
- Companion cells and sieve tubes are connected through structures called pit fields located in their common longitudinal walls. This connection allows for the transfer of substances and communication between the two types of cells.

4. **Function of Companion Cells**:
- The primary function of companion cells is to maintain the pressure gradient in the sieve tubes. This is crucial for the flow of phloem sap, which contains sugars and other nutrients.

5. **Conclusion**:
- Both the assertion and the reason are true. The assertion correctly identifies the importance of companion cells for the functioning of sieve tubes, and the reason accurately describes how they maintain the pressure gradient necessary for nutrient transport.

### Final Answer:
- Both the assertion and reason are true, and the reason correctly explains the assertion. Therefore, the answer is option A.