a. The force applied would be equal to the frictional
force.
F = us Fn
where, F = applied force = 35 N, us = coeff of static
friction, Fn = normal force = weight
35 N = us * (6 kg * 9.81 m/s^2)
us = 0.595
b. The force applied would now be the sum of the
frictional force and force due to acceleration
F = uk Fn + m a
where, uk = coeff of kinetic friction
35 N = uk * (6 kg * 9.81 m/s^2) + (6kg * 0.60 m/s^2)
uk = 0.533
They would all strike the ground at the same time. If air resistance is neglected for example in an airless tube, then gravity is the only force acting on them and it has the same effect on them dropping so the drop at the same speed.
Energy is (transferred) when it is moved from one obect to another.
The answer is (b)