D for me.
We need 'have' because the action of not seeing any evidence and not personally witnessing has been going on in his forty years of experience. Usage of 'had' will be incorrect here because 'had' means the action has already happened before some other action (which is not the case here).
Also, adverb --> modifier to a verb, an adverb, a clause, a phrase, a sentence, an adjective.
'personally' should be a modifier to 'witnessed'
I have seen very little evidence of the expression of the authentic self in American corporations; I won't deny its existence but I had personally not witnessed it once in my forty years of experience.(A) I have seen very little evidence of the expression of the authentic self in American corporations; I won't deny its existence, but I
had personally not witnessed it once in my forty years of experience.
(B) I
had seen very little evidence of the expression of the authentic self in American corporations; I won't deny its existence but I have not personally witnessed it once in my forty years of experience.
(C) I have seen very little evidence of the expression of the authentic self in American corporations; I won't deny its existence but I have
personally not witnessed it once in my forty years of experience.
(D) I
have seen very little evidence of the expression of the authentic self in American corporations; I won't deny its existence but I
have not personally witnessed it once in my forty years of experience. -
CORRECT(E) I have seen very little evidence of the expression of the authentic self in American corporations; I won't deny its existence but I
will have not personally witnessed it once in my forty years of experience.