Question

If a skydiver weighs 800 N, what would air resistance be when he is at terminal velocity?

Answer 1

Let his weight be mg.

For a falling body in a fluid.

W - V - U = ma.    Where
                             W = Weight = 800N.
                              m = mass
                             V = Viscous Force = Air Resistance.
                             U = upthrust = Negligible for air.
                             a = acceleration
                             g = acceleration due to gravity.
Note also at terminal velocity  a = zero.
 
W - V - 0 = m(0)
800 - V  = 0
800 = V. 

Therefore the viscous force which is equivalent to air resistance is also 800N.
Cheers.

Answer 2

We call "terminal velocity" the constant speed of a falling body
when it is no longer accelerating.

We know that if a body is not accelerating, then the net force
on it is zero.

From the question, we know that the downward force of gravity
on the skydiver is 800 N.

If the 800 N downward plus the air resistance upward add up
to zero, then the air resistance upward must also be 800 N.