Choice "B" is correct. Any direct financial interest in a client impairs independence, even if it is immaterial.
Choice "d" is incorrect. A material illegal act may require disclosure in or adjustment to the financial statements, whereas an immaterial illegal act may not require disclosure.
Choice "a" is incorrect. A material weakness in internal control will affect the nature, timing, and extent of audit procedures, whereas an immaterial weakness in internal control may have little impact on the audit.
Choice "c" is incorrect. An auditor is likely to use positive confirmations for material accounts receivable, but may consider negative confirmations for immaterial receivable balances.