Why Do Many H II Regions Glow with a Prominent

Why do many H II regions glow with a prominent red color?

A) Ionized hydrogen, like neutral hydrogen, has the Hα line as one of its electron transitions.
B) Ionized hydrogen does not emit a red line, but it has a yellow line and a magenta line, and the combination of these produces the sensation of red.
C) We cannot detect ionized hydrogen directly, so we use CO as a tracer. And CO produces a red line.
D) The free protons and electrons which make up the plasma of an H II region can reunite briefly to form a hydrogen atom. In the process the red Hα line is emitted.