Which of the following is a characteristic of electromagnetic waves?

they can travel with or without a medium

hope this helps :))

it may be wrong but i think its 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.