He was know for his honesty

"He was known for his honesty" is the correct phrase, but the question appears to contain a typo with "know" instead of "known".

The word that best fits in the context is "complete", making the sentence: "He was complete in his honesty."

"Complete honesty" is a common collocation, meaning someone is fully or entirely honest.

Other options do not fit grammatically or idiomatically:

"Real honesty" sounds awkward and is not commonly used.

"Utter honesty" is grammatically possible but less natural than "complete honesty".

"Throughly" is incorrect; the correct form would be "thoroughly", which is an adverb and doesn't fit here.