To account for increasing the Allowance for doubtful debts by £4,935:
(a) Debit Bad debts account £4,935, credit Allowance for doubtful debts account £4,935
(b) Debit Bad debts account £4,200, credit Allowance for doubtful debts account £4,200
(c) Debit Bad debts £4,935, credit VAT account £735, credit Allowance for doubtful debts £4,200
(d) Debit Bad debts £4,200, debit VAT account £735, credit Allowance for doubtful debts £4,935