Explain why when landing on a firm surface after a fall you should not land with stiff legs.

While lending on a surface, we should bend our knees

to provide more time to the velocity change thus decresing the net force on the legs.

transfers a part of reactionary force to thick thigh and calf muscles whicch act as dampers absorbing a part of energy and provides cushioning effect.

Explanation:

We know that Force equal Mass times acceleration.

Acceleration equals the change in velocity per unit time. Hence if the time involved in velocity change is large, resultant acceleration would be less. Hence the resulting force would also be of less magnitude.

The above explanation owes to the act of bending of knees while jumping.

When we jump and simultaneously land, we fall with a force equalling “mass of our body * gravity”. Upon our landing, on the ground, the reactionary force is transmitted to our legs.

Considering our legs are stiff and straight, the force travels quickly through the bones and muscle without sufficient change in the velocity. Hence, we experience nearly the same force on our bones as a result of which bones some-times buckle.

A safer way to land is Landing while folding of the legs. By folding our legs, we give force extra time to travel through the bended section of knee bones. During the travel, the force interacts with thick thigh and calf muscles which act as dampers and absorbs some amount of force.  

By bending, we also increase the time thus decreasing acceleration, hence the force on legs. Thus, by the dual effect of increasing time and damping effect out legs are saved from the excessive buckling effect and remain safe.

there are no contact forces mentioned on the list of choices that you've provided.