How Buoyancy force on an object is related to the weight of liquid it displaces?

The buoyancy force (B) is equal to the weight (W) of the fluid that a body in that fluid displaces. Fluid pressure increases with depth because of the (gravitational) weight of the fluid above. This increasing pressure applies a force on a submerged object that increases with depth. The result is buoyancy

volleyball players can not touch the net while the ball is in play

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.