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 beasing 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 statement one by one to determine their correctness.

### Step-by-Step Solution:

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

2. **Analyzing Statement (a)**: "Companion cells maintain the osmotic balance in sieve elements."
- This statement is **correct**. Companion cells help regulate the osmotic pressure within sieve tubes, which is essential for the proper functioning of phloem transport.

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

4. **Analyzing Statement (c)**: "Sieve tubes are associated with companion cells in all seed-bearing plants."
- This statement is **incorrect**. While companion cells are present in angiosperms (flowering plants), they are absent in gymnosperms. In gymnosperms, sieve cells are associated with albuminous cells instead of companion cells.

5. **Analyzing Statement (d)**: "Companion cells are present in Pinus only."
- This statement is **incorrect**. Companion cells are not present in Pinus (a gymnosperm). Instead, gymnosperms have sieve cells and albuminous cells. Companion cells are found in angiosperms.

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

Thus, the correct answer is that only statements A and B are correct.

### Final Answer:
The correct option is: **Only A and B are correct.**