Ais a small group of atoms within a molecule that governs how it interacts with other molecules.

Ithink the answer is b.

Ineed a picture or something 

It is kept in an engine.hope it

When the electrons fall back down and leave the excited state, energy is re-emitted in the form of a photon. the wavelength (or equivalently, frequency) of the photon is determined by the difference in energy between the two states. these emitted photons form the element's spectrum.