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.

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the correct option is c.

isotopes refers to different forms of the same element with the same atomic number but different mass number as a result of difference in their neutron numbers. isotopes have different relative atomic mass but share the same chemical properties.