A monochromatic source emits light of frequency $\nu$ and total radiated power $P$. The number of photons emitted per second by the source is

1. Each photon carries energy $E_{\text{ph}} = h\nu$. 2. Power is energy emitted per unit time. If $N$ photons are emitted per second, then $P = N \cdot E_{\text{ph}} = N\,h\nu$. 3. Solve for $N$: $N = \dfrac{P}{h\nu}$. 4. Dimensional check: $\dfrac{[\text{J/s}]}{[\text{J}]} = [\text{1/s}]$, the right units for a rate. 5. Options B and C have $P$ in the numerator multiplied (wrong dimensions). Option D is the inverse — gives time-per-photon energy, not photons-per-second. _Source: NCERT Class 12 Physics Part 2, Ch 11, Example 11.1 (part b) — same trick used to find photons/s from a laser power, p. 10._