Step 1: Understand the meaning.
The word 'Illegal' means something that is not allowed by law, unlawful.
Step 2: Identify the antonym.
The opposite of 'illegal' is 'lawful,' which means allowed or permitted by law.
Step 3: Eliminate incorrect options.
(A) Great – unrelated.
(B) Minor – refers to size/importance, not legality.
(D) Follower – unrelated.
Thus, (C) is correct.