Key Concept: Induced breeding needs a hormone that can trigger the fish's OWN spawning hormone release.
Why it's correct: Synthetic GnRH analogues (Ovaprim, Ovatide) stimulate the pituitary to release gonadotropins, inducing spawning in Indian major carps outside their natural breeding season.
Why others fail: Insulin and thyroxine have no reproductive-hormone role, and melatonin only influences seasonal cycles — none directly triggers spawning the way GnRH does.