What factors determine the number of hand brakes to be applied when setting out cars on a grade?

The correct choice is based on understanding how environmental and physical conditions influence the stability of rail cars on a grade. When setting out cars on a grade, the primary factors to consider include the grade's steepness, the level of adhesion between the wheels and the rail, and weather conditions that could affect traction.

The grade of the track is critical because steeper grades require more hand brakes to prevent cars from rolling back or moving unpredictably once they are uncoupled. Adhesion refers to the friction that holds the wheels on the rails; when this is lower—perhaps due to rain, snow, or ice—more hand brakes need to be applied to ensure safety. Additionally, adverse weather can affect how effectively the brakes will hold the cars in place, making it necessary to adjust the number of hand brakes accordingly.

Other options may consider some relevant factors but do not encompass the key aspects that directly affect the application of hand brakes when managing cars on a grade. For instance, while train speed and load weight are important in overall train handling, they aren't as crucial for deciding specifically how many hand brakes are needed on a grade. Understanding the combination of grade, adhesion, and weather conditions helps ensure the safe operation of trains, especially in challenging environments.