Choose the most appropriate antonym for the underlined word. The judge thought…

An antonym is a word that expresses the opposite meaning of a given word. The adjective 'deliberate' means done consciously and on purpose -- planned or premeditated, rather than accidental.

In the sentence, 'the offence was deliberate' means the offence was committed knowingly and on purpose. Its antonym must express that the act happened WITHOUT intent -- that is, by accident.

• Casual means informal or relaxed in manner; it says nothing about whether an act was planned, so it does not oppose 'deliberate.'

• Legitimate means lawful or officially valid; it concerns legality, not intent, so it does not oppose 'deliberate.'

• Negligible means insignificant in amount or degree; it concerns magnitude, not intent, so it does not oppose 'deliberate.'

• Unintentional means done without conscious intent -- exactly the opposite of 'deliberate' (planned, on purpose).

Cross-check by substitution: 'The judge thought that the offence was unintentional' reads coherently as the logical opposite of the original sentence, and standard dictionaries (Oxford, Cambridge) list 'unintentional' as a direct antonym of 'deliberate.'

Hence, the correct antonym for 'deliberate' is 'unintentional.'