Final answer:
The normal force on an object with mass m at rest on the floor is N = mg, acting in the upward direction, counteracting the downward pull of gravity.
Step-by-step explanation:
When an object with mass m is at rest on the floor, the magnitude of the normal force acting on it is equal to the object's weight. The correct expression for the normal force in this case is N = mg, where g is the acceleration due to gravity. This force acts in an upward direction because it is a reactive force that opposes the weight of the object, which pulls it downward due to gravity.
Considering Newton's third law of motion, the surface exerts this normal force on the object that is equal in magnitude and opposite in direction to the object's weight. Thus, the correct option for the magnitude and direction of the normal force is b. N= mg in upward direction.