How does friction act as an unbalanced force for objects in motion?

Friction is a force that acts in a direction opposite to motion. If there were not force of friction, a baseball player would never stop and would be continuously moving in the direction of motion. Balanced forces do not cause a nonmoving object to start moving.

a and c

c is 100% but i am confused in a

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.