The answer is A). The entry should have been, Dr Purchases $50 Cr Payables $50
As Purchases are debited with $50, hence the expenses are increased by $50, while Payables aren’t credited (since its omitted from A/P), meaning Liabilities aren’t increased by $50. As Expenses are on the debit side of TB and Liabilities are on the credit side of TB, hence the Debit side is $50 less than the Credit side.