What is the unit of solid angle?

The correct answer is Option 3. 

Key Points

•  The unit of solid angle is the steradian (sr).
• One steradian is defined as the solid angle that, from a point, subtends an area on the surface of a sphere equal to the square of its radius.
• The steradian is a dimensionless unit, and it is used to measure the extent of a three-dimensional angle or the spread of a beam of radiation or light.
• For example, a sphere encloses 4π steradians of solid angle, while a cone with a circular base that subtends an angle of one radian at the apex encloses 1 steradian of solid angle.