To solve the question about the unit of solid angle, let's start by understanding the basic concepts related to solid angles and their units.

Concept of Solid Angle: A solid angle is a measure of how large a particular object appears to an observer looking from a specific point. In simpler terms, it describes the two-dimensional sector of the three-dimensional space around a point. It is analogous to how an ordinary angle relates to a circle or plane. Solid angles are crucial in fields like physics and astronomy for describing the apparent size of an object as seen from a particular point.

Unit of Solid Angle: The unit of solid angle in the International System of Units (SI) is the steradian, abbreviated as sr. The steradian is defined in terms of a sphere: a solid angle of one steradian subtends an area of (r^2) on a sphere of radius r. Since the total surface area of a sphere is 4 \pi r^2, a complete sphere around a point involves a solid angle of 4 \pi steradians.

Analysis of Options:

• Solid angle: This is a concept, not a unit.
• Radian: This is the unit for measuring plane angles, not solid angles.
• Steradian: This is the correct unit for measuring solid angles.
• Candela: This is the SI unit of luminous intensity and is unrelated to angles.

Conclusion: The correct answer is steradian, as it is the standard unit used to measure solid angles. Among the given options, only steradian is an appropriate measure for this type of angle in three-dimensional space.