What Causes the Characteristic Red Color of an Emission Nebula

What causes the characteristic red color of an emission nebula?

A) thermal (blackbody) radiation with its peak in the visible red part of the spectrum
B) electrons jumping from the n = 2 energy state to the n = 3 energy state in hydrogen atoms
C) electrons jumping from the n = 3 energy state to the n = 2 energy state in hydrogen atoms
D) electrons jumping from the n = 2 energy state to the n = 1 energy state in hydrogen atoms