What is convection? does it only affect plate tectonics?

Convection is the warming and rising of warm matter, and the cooling and falling of colder matter. This applies to melted rocks in plate tectonics, water currents in the ocean, and wind currents in the atmosphere.

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.