Recall the SI definition of candela

The candela is defined as:

the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency $540 \times 10^{12}\ \text{Hz}$ and that has a radiant intensity in that direction of $\frac{1}{683}\ \text{W sr}^{-1}.$

Compare with the given form

The question gives:

• frequency $= A \times 10^{12}$ Hz
• radiant intensity $= \dfrac{1}{B}$ watt per steradian

From the SI definition: $A = 540$ and $\frac{1}{B} = \frac{1}{683} \implies B = 683$

Match with the options

The pair $(A,B)$ is: $(540,683)$

So the correct option is C.

Comparison with stored answer

Stored correct answer: C

Our derived answer: C

Hence, the derived answer agrees with the stored answer.