Still my shaky knowledges, I apologize again for this.
Part A:
Force is halved, Mass remains unchanged, Acceleration is halved.
So that a = 5 m/s2
Part B:
Force is constant, mass is halved, acceleration is doubled.
Therefore a = 20 m/s2
Part C;
Force is halved, Mass is halved, therefore acceleration is constant.
Therefore a = 10 m/s2
Part D:
Force is halved, Mass is doubled, Acceleration is halved twice <---- which is 1/4.
Therefore a = 5 x 1/4 = 1 1/4 m/s^2