Choice "D" is correct. Audit evidence obtained from independent sources outside of the entity is more reliable.
Choice "b" is incorrect. Audit evidence obtained from independent sources outside the entity is more reliable that internal evidence.
Choice "a" is incorrect. Audit evidence obtained directly by the auditor (observation) is more reliable than evidence obtained indirectly (inquiry about the application of a control).
Choice "c" is incorrect. Audit evidence obtained from independent sources outside the entity is more reliable than internal evidence. Note that the multiple internal sources could be in collusion.