According to Wikipedia, the sea trout is the anadromous form of the brown trout. What's anadromous, I hear you ask. Well: an anadromous fish is one that migrates from the sea into fresh water to spawn – as distinct from a catadromous one, which migrates from fresh water into the sea to spawn, or a potamodromous one, which migrates from lakes into rivers and streams to spawn.
Most brown trout are potamodromous, apparently; and it's only the anadromous type that's known as the sea trout, sewin, peal (etc.).
Given all this, I would suggest that if you're setting this as a quiz question, you should accept "brown trout", or maybe even just "trout", as a correct answer.
© Haydn Thompson 2017