Answer: A. The magnetic field strength would increase.
Step-by-step explanation:
From ohm's law:
I ∝ V
When, voltage increases, current increases.
And in an electromagnet, strength of magnetic field increases with increase in current (I) and number of coil (n).
B = μ n I
Thus, when battery is changed, the magnetic field strength would increase because amount of current flowing in the conducting coiled wire.