The correct answer is option 'C' which is the ruling gradient.

The ruling gradient is the maximum allowable gradient or slope that a locomotive can haul a maximum load on a specific section of a railway track. It is determined based on various factors such as the power of the locomotive, the weight and type of the train, and the track conditions.

Here is a detailed explanation of why the ruling gradient determines the maximum load that can be hauled by a locomotive on a section of track:

1. Definition of the ruling gradient:
- The ruling gradient is the steepest gradient or slope on a section of track over which a locomotive can haul the maximum load specified for that section.
- It is designed to ensure that a locomotive can safely and efficiently haul the maximum load without causing any damage to the equipment or compromising safety.

2. Factors influencing the ruling gradient:
- Power of the locomotive: The ruling gradient depends on the tractive effort or pulling power of the locomotive. A more powerful locomotive can handle steeper gradients.
- Weight and type of the train: The ruling gradient also considers the weight of the train and the type of cars being hauled. Heavier trains or trains with a higher proportion of loaded cars may require a less steep gradient.
- Track conditions: The ruling gradient takes into account the condition of the track, including the alignment, grade, curvature, and track maintenance. A poorly maintained track may require a less steep gradient to ensure safe operation.

3. Determining the ruling gradient:
- The ruling gradient is determined through engineering calculations and analysis. It is based on the locomotive's tractive effort, the weight and type of the train, and the track conditions.
- The calculations consider the maximum tractive effort available at the locomotive's driving wheels, the resistance offered by the train, and the gradient of the track.
- The ruling gradient is typically expressed as a percentage or ratio, indicating the rise in elevation over a given horizontal distance (e.g., 1 in 100 or 1%).

4. Importance of the ruling gradient:
- The ruling gradient is essential for ensuring the safe and efficient operation of trains on a railway network.
- It helps determine the maximum load that a locomotive can haul on a specific section of track, allowing for proper planning and scheduling of train operations.
- By adhering to the ruling gradient, the risk of locomotive stalling, excessive wear on equipment, and potential accidents due to inadequate braking or control is minimized.

In conclusion, the ruling gradient is the maximum allowable gradient on a section of track that a locomotive can haul a maximum load. It is determined based on factors such as locomotive power, train weight, and track conditions to ensure safe and efficient train operations.