D: Half
We know that,
F = m.a, where F is the force, m is the object's mass and a is the acceleration.
In the first case, we observed that a1 = F/m.
In the second case, we observed that the mass has been doubled, so a2 = F/2m .
By the ratio of the two cases, we get
a1/a2 = F/m / F/2m
or, a1/a2 = 2
or, a1 = 2.a2
or, a1/2 = a2
Therefore, the acceleration gets half of it's original measurement.
