When We Speak of the "Spectrum" of a Radiation Source

When we speak of the "spectrum" of a radiation source, we are referring to

A) the set of velocities that its particles actually have.
B) the overall range of velocities that its particles could possibly have.
C) the various types of atoms and molecules of which the source is composed.
D) the various frequencies at which its particles collide with each other.
E) the set of frequencies that the source can emit.