A cotter joint is used to connect two __________ rods.

Answer: Option A

A cotter joint is a type of mechanical connection used to connect two rods together.

1. Co-axial Rods:
The cotter joint is specifically designed to connect co-axial rods, meaning the rods are aligned along the same axis. This ensures that the rods remain in alignment and can transmit motion or force effectively along the same line.

2. Perpendicular Rods:
If the rods were perpendicular to each other, a different type of joint would be required, such as a pin joint or a universal joint. The cotter joint is not suitable for perpendicular rods.

3. Parallel Rods:
Parallel rods may be connected using other types of joints, but not a cotter joint, as this joint is meant for rods that are co-axial.

Therefore, the correct answer is Option A: Co-axial.