The correct answer is D. smooth and wet.
The coefficient of friction is a measure of how much two surfaces stick together. It is calculated by dividing the force required to slide one surface over the other by the normal force, which is the force pushing the two surfaces together.
The coefficient of friction is lower when the surfaces are smooth and wet. This is because the water molecules between the surfaces act as lubricants, reducing the friction between them.
When the surfaces are rough, there are more points of contact between them, which increases the friction. When the surfaces are dry, there is no water to act as a lubricant, so the friction is higher.
Therefore, the coefficient of friction is lowest when the surfaces are smooth and wet.