。
She must be very busy.
Literal
She [topic-は] very busy [must be-にちがいない].
~にちがいない expresses the speaker's strong inference — 'there's no doubt that, must be.' Stronger than ~だろう or ~かもしれない, it conveys high confidence based on evidence rather than speculation. The kanji form ~に違いない is identical in meaning. Often paired with adverbs of certainty (きっと, 絶対) for emphasis.