Which of the following is the physical quantity for the expression arc/radius ?

Plane angle 

The correct answer is option 4. Plane angle.

The physical quantity for the expression arc/radius is Plane angle.

The expression arc/radius represents the measurement of an angle in radians. In geometry, an angle is often measured in terms of the ratio of the length of an arc to the radius of the circle that the arc is a part of. This measurement is known as a radian and is a unit of plane angle. Therefore, the expression arc/radius represents a measurement of plane angle.