What Color Is an H I Region

Multiple Choice

What color is an H I region?

A) red from ionized hydrogen's alpha transition, such as in the Orion Nebula
B) red from reddening due to dust
C) colorless; it emits in the radio region
D) blue from the scattering of light off the tiny atoms, such as around the Pleiades
E) blue from ionized helium's alpha transition, such as the Horsehead Nebula