Answer:
Option b
Step-by-step explanation:
The formula of momentum is 

where p is the momentum , m is the mass in kg and v is the velocity in m/s, so 
a) momentum is
 

b) momentum is

c) momentum is 
here since the rock is sitting on top of a hill which means the object is stationary or in other words the object is at rest, which means if an object is at rest the velocity is 0m/s so momentum is zero of a stationary object because 

d) momentum is 

so the highest momentum is in part b which is 12 so option b is our answer.