A

Step 1: Consider the forces acting on the man. He experiences the gravitational force downward and the normal force from the floor of the bus upward.
Step 2: The gravitational force on the man can be expressed as \( F_g = mg \), where \( m \) is the mass of the man and \( g \) is the acceleration due to gravity.
Step 3: As the bus is accelerating with constant acceleration \( a \), the man will experience a pseudo-force acting in the opposite direction of acceleration. This pseudo-force can be expressed as \( F_p = ma \).
Step 4: The net force acting along the vertical direction, considering the pseudo-force horizontally, will not affect the normal force. Hence, the normal force (force on the man due to the floor of the bus) must still balance the weight of the man to prevent him from falling.
Step 5: Therefore, the normal force \( N \) is equal to the weight of the man, which can be stated as: \( N = mg \).
Thus, the force on the man due to the floor of the bus is \( mg \), which shows that the normal force does not change due to the horizontal acceleration of the bus.
Therefore, the answer is \( mg \) or according to the force per unit mass, we say that it remains equal to the gravitational force acting on the man.