1. Energy of one photon

For monochromatic light, energy of each photon is $E = h\nu$ where $h = 6.63 \times 10^{-34}\,\text{J s}, \qquad \nu = 6 \times 10^{14}\,\text{Hz}$

So, $E = (6.63 \times 10^{-34})(6 \times 10^{14})$ $E = 39.78 \times 10^{-20}$ $E = 3.978 \times 10^{-19}\,\text{J}$

2. Use power to find number of photons emitted per second

Power is energy emitted per second: $P = 2 \times 10^{-3}\,\text{W} = 2 \times 10^{-3}\,\text{J/s}$

If $n$ photons are emitted per second, then $nE = P$ Hence, $n = \frac{P}{E} = \frac{2 \times 10^{-3}}{3.978 \times 10^{-19}}$

$n = \frac{2}{3.978} \times 10^{16}$ $n \approx 0.503 \times 10^{16}$ $n \approx 5.03 \times 10^{15}$

3. Match with options

$n \approx 5 \times 10^{15}$

So the correct option is A.