Fish plates, also known as joint bars or splice bars, are used in rail joints to serve several important functions. The correct answer, option 'D', includes the following functions:

1. Maintain the continuity of rails:
Fish plates are used to join two rails together at the rail joints, ensuring the continuity of the rail track. They provide a secure connection between the two rails, preventing any separation or misalignment that could disrupt the smooth movement of trains. By maintaining the continuity of rails, fish plates help to ensure a safe and efficient railway system.

2. Provide for any expansion or contraction:
Railway tracks are subjected to temperature variations, which cause the rails to expand or contract. Fish plates are designed to accommodate these thermal movements. They allow the rails to expand or contract without exerting excessive stress on the rail joints. The fish plates have holes that are slightly larger than the rail bolts, allowing for some movement and preventing the rails from buckling or warping due to thermal stresses.

3. Transfer the load to the ballast:
The load from the trains traveling on the rails is transferred to the ballast, which is the layer of crushed stones or other materials beneath the railway track. Fish plates play a crucial role in distributing the load evenly across the rail joints and transferring it to the ballast. They help to prevent excessive stress concentration at the joints, which could lead to rail failure or track instability.

4. Maintain correct alignment:
Fish plates help to maintain the correct alignment of the rails at the joints. They ensure that the two rails are properly aligned and in the same horizontal and vertical plane. This alignment is important for the smooth and safe passage of trains, as any misalignment can cause derailments or accidents. Fish plates are designed to provide a secure and stable connection between the rails, minimizing any potential misalignment.

In conclusion, fish plates are essential components in rail joints as they maintain the continuity of rails, provide for expansion and contraction, transfer the load to the ballast, and maintain correct alignment. These functions are crucial for the safe and efficient operation of railway tracks.