If the toy helicopter weighs 115 N and is hovering motionless in the air, which of these options MOST accurately describes the force that the helicopter's propeller must exert?

If the helicopter is not moving, then it is not being accelerated.

By second Newton's law we have that:

F = m*a

Force equals mass times acceleration.

Then if we do not have acceleration, we have:

F = m*a = m*0 = 0.

Then the total force must be zero.

So if the gravitational force (pulling down) is 115N, then the force of the propeller must be such that:

Total force = F - 115N = 0

F = 115N

The force of the propeller must be also of 115N, but in the opposite direction.

i would say