Off-course Komal, you are absolutely correct in the sense that whenever the value of liability falls, then it is shown on the credit side of the Revaluation Account, as this indicates the favourable position for the firm. However, in this scenario, as you can see that there exists no bad debts (as per the balance sheet given in the questions), which means that all the debtors of Rs 19,000 are good. But in the adjustments, we are directed to write-off bad debts of Rs 2,000. This implies that out of the total debtors of Rs 19,000, debtors of Rs 2,000 are bad now. This automatically means that the balance of debtors have fallen (or bad debts have risen). In other words, we can say that firm's liabilities have risen. Thus, bad debts are shown on the debit side of the Revaluation Account.
Note- It should not be assumed that the word 'written-off' here implies that the bad debts have fallen, in fact, it implies that bad debts have risen.