From IB physics May 2017 - Physics question

The answer is C)
Can someone explain this to me how to tackle this kind of problems, having friction?


dynamic friction Fk=µkN with µk=coefficient of dynamic friction
Fk=0,3 * mg = 0,3 * 10 = 3N
so 3N is the force of the block on the trolley

From Newton’s 3rd law of motion, every action creates an opposite and equal reaction, we can say that 3N is also the force of the trolley onto the block

For the trolley:
3=4 * a
a=3/4 = 0,75