Sieve tubes and companion cells are sister cells and
(a) Companion cells maintain the osmotic balance in sieve elements
(b)Both cells are derived from same mother cells
(c )Sieves are associated with companion cells in all seed bearing plants
(d) Companion cells present in Pinus only

Options

A. Only (a) and (b) are correct
B. (a), (b) and (c ) are correct
C. (a), (b) and (c ) are incorrect
D. Only (a) and (c ) are correct

Answer

To solve the question regarding sieve tubes and companion cells, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding Sieve Tubes and Companion Cells**:
- Sieve tubes are part of the phloem tissue in plants, responsible for the transport of nutrients, particularly sugars. Companion cells are closely associated with sieve tubes and play a crucial role in their function.

2. **Analyzing Option (a)**:
- **Statement**: "Companion cells maintain the osmotic balance in sieve elements."
- **Evaluation**: This statement is **true**. Companion cells help in maintaining the osmotic pressure and nutrient flow within the sieve tubes.

3. **Analyzing Option (b)**:
- **Statement**: "Both cells are derived from the same mother cells."
- **Evaluation**: This statement is also **true**. Sieve tubes and companion cells originate from the same precursor cell during development, which is why they are referred to as sister cells.

4. **Analyzing Option (c)**:
- **Statement**: "Sieves are associated with companion cells in all seed-bearing plants."
- **Evaluation**: This statement is **false**. While angiosperms have companion cells associated with sieve tubes, gymnosperms do not have companion cells; they have sieve cells instead. Therefore, this statement is incorrect.

5. **Analyzing Option (d)**:
- **Statement**: "Companion cells present in Pinus only."
- **Evaluation**: This statement is **false**. Pinus is a gymnosperm, and gymnosperms do not have companion cells. Thus, this statement is also incorrect.

### Conclusion:
Based on the evaluations:
- Options (a) and (b) are correct.
- Options (c) and (d) are incorrect.

The correct answer is that **only options (a) and (b) are correct**.