The irradiance of the sun is primarily dependent on the wavelength of the radiant energy it emits. This is because the irradiance, or the power per unit area, varies with wavelength due to the sun's emission spectrum, which is shaped by its temperature and other physical properties.
While the number of sunspots can influence solar irradiance to some extent, the primary factor among the options provided is the wavelength of the radiant energy emitted by the sun.