ANS:A - the number of turns is decreased

Inductance (Physical property of a coil) L = U.N2.l.A.

U = Permeability of core,
N2 = NxN (Number of turns).
L = Length of coil.
A = Cross sectional area of coil.

So if to decrease L.

You decrease (remove of iron core).

Reduce N turns of coil.
Reduce length of coil.

The correct answer is not given in the question.