Choice "D" is correct. Evidence concerning proper segregation of duties is generally obtained through inspection and observation.
Choice "a" is incorrect. Completing an internal control questionnaire might aid the auditor in understanding how controls are supposed to work, but it does not provide evidence with respect to whether controls are actually functioning as designed.
Choice "c" is incorrect. Performing substantive tests to verify details about a bank balance provides evidence as to whether the balance is fairly stated, not whether controls surrounding the account are operating effectively.
Choice "b" is incorrect. Preparing a flow chart of the duties performed and the entity's available personnel might aid the auditor in understanding how controls are supposed to work, but it does not provide evidence with respect to whether controls are actually functioning as designed.