This is difficult. I wonder how many different answers you will get? A, B, and C, are wrong. They all run ideas together that need to be broken apart. We could make B correct if we could put a comma after the word "however." I think it's D. I thought it was E at first, but in order to make E work I think we need to change the comma after the word "that" to a semicolon: E, corrected: “You’re highly experienced and efficient. I know that; however you are not tolerant, which is essential for this job." Even then, I don't really like it, because it feels wrong to put "I know that" in the second sentence, when the thing that he knows is back there in the first sentence. I can make D sound logical by imagining that the candidate says "Why did you reject me? I've shown you that I'm highly experienced and efficient." The hiring manager could answer in two sentences: "You are highly experienced and efficient, I know that. However, you are not tolerant, which is essential for this job." Or, he can combine the two thoughts together into a single sentence by replacing the period with a semicolon. Notice that depending on how we punctuate the sentence, when the hiring manager says "I know that," he can be saying he knows that the candidate is experienced and efficient, or he can be saying that he knows that the candidate is not tolerant.

I believe the correct answer is E.