Ionization of Hydrogen in H II Regions Is Most Visible

Ionization of hydrogen in H II regions is most visible at

A) 12,367 nm, the Paschen alpha line.
B) 656.3 nm, Balmer alpha line for 3-2 electron transition.
C) 21 cm in the radio region for the spin-flip of its electron.
D) 121.3 nm, the Lyman alpha ultraviolet line for 2-1 electron transition.
E) 473.2 nm, the blue-green emission line for 4-2 electron transition.