d
The sodium spectrum is dominated by the bright doublet, which are known as the sodium \(D\) lines. The wavelength of these lines are around the range \(588.9\; n m\) and \(589.5 \;n m\). Now the wavelength of these sodium \(D\) lines falls under the wavelength range of the X rays in the spectrum. So we can say that the doublet of sodium belongs to the X rays.

Most of the hydrogen gas in the interstellar medium is in cold atomic form or in the molecular form. They emit a wavelength due to a slight energy change in the hydrogen atoms. This wavelength is \(21.1 cm\) and so it is called the \(21\;cm\) line radiation. This wavelength lies in the spectrum of the visible light. Hence the wavelength emitted by atomic hydrogen in interstellar space belongs to the visible light spectrum.

The difference in the energy of two close energy levels in a hydrogen atom is called lamb shift. The wavelength

corresponding to this difference in energy is around \(30 \;cm\). This is in the range of the microwaves.