You said it exactly.
C also seemed to address the problem that most accidents are caused by human error.
That does not mean that the likelihood of human error cannot be lessened by thoughtful design of the consoles.
We're looking for something that is "strongly supported." It's in your interest to think of these as asking for what "must be true." Just maybe be a little less strict with the language.
We know that "such everyday events cannot be thought unlikely to occur over the long run."
(A) is too strong. We don't know that the threat is "ever-increasing."
(B) is perfectly weak. "Not improbable" can be easily justified.
(C) is too strong. We cannot establish that the threat "cannot be lessened."
(D) is out of scope. We don't know this about the design of nuclear power plants.
(E) is too strong. We cannot establish that only human error will cause future accidents.
Thanks for the great question. The language in the answer choices is very informative for looking at inference questions.