The force of 100N acts on a body moving with constant velocity of 20m/s on a floor in a strainght line. what is the force of friction between the body and the floor?

If the object's speed and direction are not changing, then

we would say that its acceleration is zero.

That in turn tells us that the net force on it is zero.

So if somebody is pushing on it with 100N of force, then

the force of friction trying to oppose its motion is also 100N,

in the opposite direction.