Concept:Revenue accounts normally have credit balances, whereas assets and expense accounts have debit balances.Explanation:Return inwards account is a contra-revenue account and has a debit balance.Machinery account is an asset account and has a debit balance.Cash account is also an asset account and has a debit balance.Sales account is a revenue account, and revenue increases on the credit side.Therefore, among the given options, only the sales account has a credit balance.Answer:C. sales account