Final answer:
The epithet of Apollo that reflected the difficult nature of his replies was the oracular epithet.
Step-by-step explanation:
The epithet of Apollo that was thought to bear testimony to the difficult nature of his replies is the oracular epithet. The oracles of Apollo, such as the famous Oracle of Delphi, were known for providing cryptic and enigmatic responses to questions. These responses often required interpretation and were considered to be challenging to understand.