The bright lines of an emission spectrum are the result of

Transition of electrons from higher energy level to lower energy level.

Explanation:

There are two types of lines in the spectrum which are given below:

i) Absorption lines or dark lines: When atom absorb photons then it comes to an excited state and electrons transition occurs from lower to higher energy level. This absorption of photons is shown with dark lines on spectrum.  

ii) Bright lines: When transition of electrons occurs from higher energy level to lower energy level, photons are emitted. The emission of photons is represented by bright lines on the spectrum.

Hence, Transition of electrons from higher energy level to lower energy level is the right answer.

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