From the options given below, choose the word which is opposite in meaning to…

Concept: An antonym is a word that carries the opposite meaning of another word. To identify the antonym of an adjective, look for an option that names the reverse condition or quality along the same dimension of meaning as the given word.

Application: 'Lawful' means permitted, recognised, or in accordance with the law. Its exact opposite is a word meaning not permitted by, or against, the law — that is 'Illegal', which specifically describes an act, item, or status forbidden by law.

Cross-check: Contrasting the other options by their own meaning confirms this:

• 'Authorised' means having official permission or sanction, which is close in meaning to 'lawful' (a near-synonym), not its opposite.

• 'Errant' means straying from proper or expected conduct — a behavioural term rather than a description of legal status.

• 'Encroach' means to intrude gradually upon another's rights or territory — a verb describing an act of overreach rather than a legal-status adjective.

'Illegal' is the only option that directly names the opposite legal status to 'lawful', so it is the correct antonym.