Seeing examples like "garam-thanda" and "sundar-kuroop", Ankit thinks Hindi antonyms are never…

Why

Ankit built a new rule from the last two examples — 'never a prefix, always a separate word' — but that's just as wrong as assuming 'always a prefix'. Both methods are genuine and standard in Hindi, and which one applies to which word is decided word-by-word, not by one overarching rule. For 'न्याय', the traditional, textbook-standard antonym is in fact formed with a prefix — 'अन्याय' ('अ-' + 'न्याय', since 'न्याय' begins with a consonant, so 'अ-' attaches directly) — and this is the fully correct, most common form. Test it: 'गरम/ठंडा' is an antonym pair of separate words, while 'न्याय/अन्याय' is a prefix-formed pair — both are standard and correct, just different methods for different words. The second option is right: 'अन्याय' is indeed the correct, standard antonym of 'न्याय' — there's no single method for forming antonyms; each word must be learned individually.