Antonyms

The task is to form the antonym of "asha" (hope). Vikram writes "anaasha", thinking adding "a-" always creates the opposite. His sister says the correct word here is different. Who is right?

Seeing words like "niraasha", Sohan thinks every antonym is formed with a prefix. So to make the antonym of "garam" (hot), he writes "agaram". His teacher says a completely different word is needed here. Who is right?

To form the antonym of "sundar" (beautiful), Priya writes "asundar", thinking it too forms with a prefix like "niraasha". Her friend says the textbook uses a different, more common word. Who is right?

Seeing examples like "garam-thanda" and "sundar-kuroop", Ankit thinks Hindi antonyms are never formed with a prefix, always a different word. So for "nyay" (justice) he looks for a completely different word like "beinsaafi", treating "anyay" as wrong. Is Ankit right?