Which of the following electrolyte can be used to obtain $$H_2 S_2 O_8 $$ by the process of electrolysis?

We need to identify which electrolyte produces $$H_2S_2O_8$$ (peroxodisulphuric acid, also known as Marshall’s acid) by electrolysis.

Peroxodisulphuric acid is formed by the electrolysis of concentrated sulphuric acid. At the anode, sulphate ions ($$HSO_4^-$$) are oxidized as shown below:

$$2HSO_4^- \rightarrow H_2S_2O_8 + 2e^-$$

In dilute sulphuric acid solutions, water is preferentially oxidized at the anode to produce oxygen. Only in a concentrated solution is the concentration of $$HSO_4^-$$ ions high enough for their oxidation to form the peroxo bond (S-O-O-S) in $$H_2S_2O_8$$.

The correct answer is Option 4: Concentrated solution of sulphuric acid.