The irradiance of the Sun primarily depends on the wavelength of the radiant energy it emits. The Sun emits electromagnetic radiation across a range of wavelengths and the intensity of this emission varies with wavelength. Additionally, factors such as the temperature of the Sun's surface also influence the spectral distribution of irradiance.
The other options provided (density of solar wind particles, energetic solar wind particles, and number of sunspots) do not directly influence the Sun's irradiance in the same way.