For some reason the new consultant treats his clients like

Plenty of great observations already on this one, but I thought I'd chime in to emphasize one particular point about comparisons. The words "like" and "as" function in very similar ways, but there is one major distinction:

"Like" must be used to compare nouns and "as" must be used to compare clauses.

Probably the first split one should notice here is the "like" vs. "as" at the beginning of the underline portion. Here we are actually trying to compare the clients directly to "idiots" (well, at least that's the comparison that the consultant seems to be making!) It's noun-to-noun, so "like" is correct and choices (B) and (D) are now out. Now that we have "like" on the mind, we should notice the second usage of it, after the comma. "Talking to them" is a clause so choice (A) is out as well.

Choice (E) is actually not an incomplete sentence. It's grammatically correct to say "For some reason he treated clients like idiots who ..." The problem here is definitely meaning. In the original, we're trying to convey that he talks to his clients as if they were mentally deficient, but in (E) the subordinate clause beginning with "who" is just modifying "idiots", saying that they actually are mentally deficient.

A subtle point, but a great example of why we should stick to grammar as long as possible!

Cheers,
Mark