Hello!
I'll try and explain why the answer is clearly not (E), but (A).
(E) actually goes against the conclusion since it states that "ONLY a manager should be held responsible for a project's failure" but the conclusion never says that. In fact, it clearly says that "but he, TOO, is to blame" so there is someone else besides him that needs to be blamed who is the contractor in this case.
So the general conclusion of the argument is that the contractor is not the ONLY one to be blamed, but ALSO the manager as well -- and the reason for this is because "he was AWARE of the contractor's typical delays and should have planned for this contingency." Answer choice (A) gives us a principle that fits pretty well with the argument since the author is assuming something along the lines of "if you are aware of the unwanted consequences of something, then you are also to blame". If we tried inserting answer choice (A) in the argument, it would look like this:
The manager was aware of the contractor's typical delays and should have planned for this contingency.
A manager should take foreseeable problems into account when making decisions. The manager is also to blame since he basically overlooked it even if he was aware of it.
I was honestly looking for an answer choice something along the lines of the assumption I made above (which includes the word "blame/responsible" like you said), but you can see that B through E are terrible answers, so (A) would be one of those answer choices that we would pick since it is the "BEST" even though we aren't 100% satisfied with.
Just for further clarification (if needed), in answer choice (A), the "foreseeable problems" would be the "awareness of unwanted consequences (which in this case is the contractor's typical delays)" and the "making decisions" would be the manager's decision to schedule the new facility's opening for Oct 17.
Let me know if you have further questions!!