In the S.I. system, the quantity measured in candela (cd) is luminous intensity. Identify the correct application of the unit 'candela'.

Introduction / Context:
S.I. assigns specific base and derived units to distinct physical quantities. Candela (cd) is one of the seven S.I. base units and plays a central role in lighting engineering, photometry, and visual ergonomics by quantifying the luminous intensity of a source in a given direction.

Given Data / Assumptions:

• The unit under discussion is candela (cd).
• Options mix unrelated quantities such as power (W), frequency (Hz), and current (A).
• We must identify the quantity for which cd is the canonical S.I. unit.

Concept / Approach:

Luminous intensity measures perceived light power in a specified direction, weighted by the standard luminous efficacy curve (photopic vision). This distinguishes it from illuminance (lux), luminous flux (lumen), radiant intensity (W/sr), or electrical power (W).

Step-by-Step Solution:

Verification / Alternative check:

In photometric relations, luminous flux (lm) = luminous intensity (cd) * solid angle (sr). This confirms cd’s role as intensity per direction, not area-based reception (lux) or energy rate (W).

Why Other Options Are Wrong:

• Electric current intensity: Measured in ampere (A).
• Power: Measured in watt (W).
• Frequency: Measured in hertz (Hz).
• Illuminance: Measured in lux (lx), not cd.

Common Pitfalls:

• Mixing luminous intensity (cd) with luminous flux (lm) or illuminance (lx).
• Confusing radiometric units (W, W/sr) with photometric units (lm, cd) that include human eye response.

Final Answer:

luminous intensity