C: 300THZ. The formula is $\Delta f = c * \Delta \Lambda / \Lambda^{2}$ where ∆f is the bandwidth range, c is the light speed, $\Delta \Lambda$ is the wavelength range and $\Lambda$ is a certain wavelength.
For frequency to be in hertz and speed of light in meter/second, we want the wavelength to be in meters.
As 1 meter = 1000,000 microns, calculating using the formula you get 300 THZ.