A5.00 kilogram block slides along a horizontal, frictionless surface at 10.0 meters per second. for 4.00 seconds. the magnitude of the blocks momentum is ?
a. 200 kg m/s
b. 50.0 kg m/s
c. 20.0 kg m/s
d. 12.5 kg m/s

In this question a lot of information's are provided. Among the information's provided one information and that is the time of 4 seconds is not required for calculating the answer. Only the other information's are required.
Mass of the block that is sliding = 5.00 kg
Distance for which the block slides = 10 meters/second
Then we already know that
Momentum = Mass * Distance travelled
                   = (5 * 10) Kg m/s
                   = 50 kg m/s
So the magnitude of the blocks momentum is 50 kg m/s. The correct option among all the given options is option "b".