Which Spectral Feature Originating in a Galaxy Is Used in the Tully-Fisher

Which spectral feature originating in a galaxy is used in the Tully-Fisher relation, by measuring its width, to determine the intrinsic luminosity of the galaxy?

A) the 2.3-mm microwave emission line from the molecule CO, which is closely associated with molecular hydrogen
B) the Lyman L $\alpha$ ultraviolet line from atomic hydrogen
C) the 21-cm radio emission line from atomic hydrogen gas
D) the Balmer H $\alpha$ red line from atomic hydrogen

Correct Answer:

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