A person jumps from a height on soft sand. Which one of the...

According to Newton's second law of motion, the average force exerted on a body is equal to the rate of change of momentum, $F = \dfrac{\Delta p}{\Delta t}$.

When a person jumps from a height onto soft sand, the sand yields under the person's weight. This increases the time of impact $\Delta t$ for the person to come to rest.

Since the change in momentum $\Delta p$ remains the same (as the initial velocity just before impact and the final velocity of zero are unchanged), an increase in the impact time $\Delta t$ results in a significant decrease in the average force $F$ exerted on the person.

A smaller force reduces the likelihood of injury. Therefore, the soft sand increases the impact time.