Which electron transition represents a gain of energy?
a. from 2nd to 3rd shell
b. from 2nd to 1st shell
c. from 3rd to 2nd shell
d. from 3rd to 1st shell

The correct answer is (A) from 2nd to 3rd shell.

The explanation :

when a gain of energy is the shift of the electrons from a shell of low energy to the shell of high energy

and we have here 2nd shell is the shell of low energy, and 3rd shell is the shell of high energy.

∴ (A) from 2nd to 3rd shell is the correct answer.

because of all the things that happen around the us, many people get into arguments which sadly could get physical, but we should actually make a change for that.