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.

answer : one can design an experiment using a liquid and an additive that can be added to the liquid and observe the chemical changes in the reaction species.

one would need a beaker which is filled with a known amount of the liquid in which thermometer is placed to measure the temperature changes; after addition of the additive to the liquid solution one can note down the temperature change and find whether the reaction is endothermic or exothermic in nature. by subtracting the initial temperature value with the final temperature one can find the temperature changes.

various combinations of liquid and additive can be used. liquid in this case can be a known quantity of water and additives can be epsom salt or without epsom salt the experiment can be conducted to notice the changes. water and ice, hydrogen peroxide and dry yeast, vinegar and baking soda,etc.