please answer this question
 

Dear Student
Impulse is defined as the rate of change of momentum. Also It is expressed as the product of impulsive force and time 

In case of a person falling on the cemented floor, the reaction time is less hence the impulsive force is large 
The cemented floor being a solid and compact material does not deform on collision hence for the momentum change to occur after collision large force is applied for small interval hence increasing the impact of impulsive force
while on the other hand person falling on a heap of sand reaction time is more thus the impulsive force is less
Particles in case of heap of sand are not intact and are deformed on collision resulting in increase in reaction time and as impulsive force is inversely proportional to time the impact of impulsive force is reduced

So both, Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Regards