What would be the total of the trial balance if a purchase return of Rs. 84 has been wrongly posted to the debit of the sales return account, but had been correctly entered in the suppliers account?

The debit side has more credits than the credit side. You need to understand the difference between the two to understand. Debit means that money is being given to you in order to purchase something or you may have received the money. Credit means that you are borrowing money from the lender with interest.

In this case, the money was wrongfully credit to the debit side which means that money has been received even if the input is correct in the account of the suppliers. This is a type of error that can be easily rectified the moment the details are discovered.