Suppose you exert 150 n on your refrigerator and push it across the kitchen floor at constant velocity. what friction force acts between the refrigerator and the floor?

Friction force is the force arising from two surfaces being pressed together and the force will depend upon the nature of the two surfaces and degree to which they are pressed. Kinetic friction is the force you exert on your refrigerator as you move it across the kitchen floor at constant velocity.

Due that the velocity is constant that means that friction force is equal to the force exert by you, otherwise the refrigerator will accelerate or decelerate and in both cases velocity will not be constant.

So then the friction force between refrigerator and floor is 150 Newtons.