In measurement of angles, identify the correct unit used specifically for solid angles in three-dimensional space, distinguishing it from plane-angle measures.

Introduction:
Angles come in two fundamental types: plane angles (2D) and solid angles (3D). Differentiating their units is essential in fields like radiometry, illumination engineering, and spherical geometry, where quantities such as luminous intensity and radiance depend on solid-angle measures.

Given Data / Assumptions:

• Plane angle units include degree, radian, and grad (grade).
• Solid angle is the 3D analogue measured over the surface of a sphere.

Concept / Approach:

Solid angle is defined as area on a unit sphere subtended by a cone or field of view. The SI unit for solid angle is the steradian (sr), whereas radians (rad) and degrees (°) apply to plane angles only.

Step-by-Step Solution:

Verification / Alternative check:

On a unit sphere, a full sphere corresponds to 4pi steradians. This confirms that steradian is the 3D coverage unit, unlike radian which measures arc per radius in a plane.

Why Other Options Are Wrong:

• Degrees/grades: Plane-angle units; not solid angles.
• Radians: Plane-angle SI unit; not for solid angles.
• Turns: Also a plane-angle measure (1 turn = 2pi radians).

Common Pitfalls:

• Assuming 'radian' also applies to 3D angles; it does not.
• Forgetting that steradian measures surface area on a unit sphere, not arc length.

Final Answer:

steradians.