The correct answer is option 'B' - mild steel canted bearing plate.

Explanation:
The purpose of a bearing plate in railway tracks is to provide a larger bearing area to the rails, especially in joints and on curves. This helps to distribute the load more evenly and reduce stress on the rails. The use of a bearing plate also helps to prevent the rail from spreading or moving sideways.

Mild steel canted bearing plates are commonly used in railway tracks for several reasons:

1. Increased bearing area: The canted design of the bearing plate provides a larger contact area between the rail and the plate. This helps to distribute the load more effectively and reduce the pressure on the rails.

2. Improved stability: The canted design also helps to improve the stability of the rail by preventing lateral movement or spreading. This is particularly important on curves, where the centrifugal forces acting on the rail can cause it to move outward.

3. Corrosion resistance: Mild steel is known for its corrosion resistance, which is crucial in railway tracks where exposure to moisture and other environmental factors can lead to rusting and deterioration. The use of mild steel bearing plates helps to ensure the longevity and durability of the track structure.

4. Ease of installation: Mild steel canted bearing plates are relatively easy to install and maintain. They can be easily fitted onto the rail joints and curves, providing a secure and stable connection between the rails.

In contrast, the other options listed in the question are not suitable for all joints and curves in railway tracks:

- Flat mild steel bearing plates do not provide the same level of stability and load distribution as canted plates. They are typically used in straight sections of track where there is less risk of lateral movement.

- Cast iron anti-creep bearing plates are designed to prevent rail creep, which is the longitudinal movement of the rail. They are not specifically designed to provide better bearing area on curves or joints.

In conclusion, mild steel canted bearing plates are the preferred choice for providing better bearing area to the rails in joints and on curves. Their canted design, increased bearing area, and corrosion resistance make them ideal for ensuring the stability and longevity of railway tracks.