Match the major interstellar gas type with the method used for observing that type. neutral hydrogen gas giant molecular clouds ionized hydrogen gas ultra-hot interstellar gas
a. radio and infrared spectral lines.
b. ultra-violet and x-ray emissions.
c. optical emissions, especially the balmer lines.
d. 21-cm wavelength emissions from electron spin-flip.

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

i will answer your question if you answer mine

because growing process had stoped after 4-5 year