に決まっている【にきまっている】

に決まっている means "it's obvious that", "of course": the speaker is so sure that there is no point discussing it. It goes after the plain form of a verb or an い-adjective, and straight after a noun or a な-adjective, with no だ in between.

• × 大変だに決まっている → 大変に決まっている

The certainty is the speaker's own, and often impatient: "obviously", "what did you expect". In conversation it is usually shortened to にきまってる決まってる.

に決まっている or に違いない

にちがいない違いない is a conclusion drawn from evidence: "it must be". It suits writing and calm reasoning. に決まっている is a flat assertion in speech, with an edge of "anyone can see that". はずだ is weaker than both: what you expect from what you know, which can turn out wrong.

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に決まっています is the polite form, but the bluntness stays. To a superior, it can sound as if you think the question was silly.