Answer:
Neutral Hydrogen gas -- 21-cm wavelength emissions from electron spin-flip.
Giant molecular clouds --- Radio and infrared spectral lines.
Ionized hydrogen gas --- Optical emissions, especially the Balmer lines
Ultra-hot interstellar gas --- Ultra-violet and x-ray emissions
Explanation:
TYPE OF INTERSTELLAR METHOD USED
GAS TYPE
Neutral Hydrogen gas -- 21 centimeter wavelength emission through electron spin-flip.
Giant molecular clouds --- Radio & infrared spectral lines.
Ionized hydrogen gas --- Optical emissions, especially the Balmer lines
Ultra-hot interstellar gas --- Ultraviolet & x-ray emissions
