Which electron transition in an excited atom results in a release of energy?

Answer is: fourth shell to the second shell.

Electrons can jump from one energy level to another, absorbing or emitting electromagnetic radiation with a frequency ν (energy difference of the levels).

When electron jump from higher to lower energy level (shell), it emitting (releasing) energy.

When the electron changes from n=4 (fouth shell)  to n=2 (second shell), the photons are emitted.