Why is the answer c for this problem?

Explanation:

If you look closely, force 1 does not reach 0.2 until 0.4 force 2 reaches 0.2 at about 0.2 - hope that made sense :P

adding a(n) to an electric circuit would increase the magnetic field around a wire;

b. solenoid

a solenoid is a type of electromagnet whose purpose is to generate a controlled magnetic field.

if the purpose of the solenoid is instead to impede changes in the electric current, a solenoid can be more specifically classified as an inductor rather than an electromagnet.