In the sentence “What can we do but sit and wait?”, the word but functions as a preposition meaning ‘except’. It shows a relationship between the action and the exception.

১. In the first option, ‘but’ is an adverb meaning ‘only’.
২. In the second option, ‘but’ acts as a relative pronoun (meaning who...not).
৩. In the third option, ‘but’ is a coordinating conjunction connecting two contrary ideas.

অতিরিক্ত তথ্য: When ‘but’ means ‘except’, it is classified as a preposition.