Why line spectrum is FINGERPRINT OF AN ATOM?

Q. An atom has x energy level, then total number of lines in its spectrum are :

Q. An atom has $x$ energy level, then total number lines in its spectrum are?

Q. Which series of line spectrum of H-atom is observed usually in both emission and absorption spectrum?

Q. In an atom an electron excites to the fourth orbit. When it jumps back to the energy levels a spectrum is formed.Total number of spectral lines in this spectrum would be

Q. An electron in excited hydrogen atom falls from fifth energy level to second energy level. In which of the following regions, the spectrum line will be observed and is part of which series of the atomic spectrum?