Heated hydrogen emits red light at exactly 656 nm: the same wavelength from a lab tube and from a distant galaxy. What fixes that number?

D. The energy gap between the $n = 3$ and $n = 2$ levels

A photon carries exactly the energy difference between two levels, and those levels are fixed by the atom. Temperature and density change how BRIGHT the line is and how many atoms are excited, never its wavelength; the container is irrelevant. That invariance is what makes spectroscopy work, the same 656 nm line identifies hydrogen anywhere in the universe.