What are the spectrum lines, and they're color emmited and nanometers

Emission lines are seen as coloured lines on a black background. Absorption lines are seen as black lines on a coloured background. The presence of spectral lines is explained by quantum mechanics in terms of the energy levels of atoms, ions and molecules.

i hope it is helpful:) mark as brainlist

i'm assuming that the correct answer is indeed the first one/ "a"..

a compound is a molecule made of atoms from different elements.

a. by the combination of an unlimited number of elements