Final answer:
Neutral hydrogen clouds are best studied using radio measurements of the 21-cm line, which allows for mapping the structure of the Milky Way and analyzing cold, nonionized hydrogen in interstellar space.
Step-by-step explanation:
Intergalactic hydrogen clouds, consisting mostly of neutral (nonionized) hydrogen, are easiest to study through radio measurements of the 21-centimeter line.
This is because hydrogen gas near hot stars, known as H II regions, are ionized and emit light, particularly the red Balmer line in the visible spectrum. However, most hydrogen in interstellar space is not ionized and lacks spectral features in the visible part of the spectrum. It is in its ground state, and thus, radio telescopes are necessary to detect the very-low-energy changes in hydrogen atoms. The 21-cm line is valuable as it allows astronomers to map the spiral structure of the Milky Way, due to its ability to pass through interstellar dust.