The answer is variant D because 'Trout are fish' Is a simple sentence and 'are fish' exprime a quality of the subject, so the verb 'are fish' meets the rules of the predicate nominative.
The word 'fish' is used as a predicate nominative in option D: 'Trout are very beautiful fish.' In this sentence, 'fish' renames the subject 'trout' after the linking verb 'are.' Therefore, D is the correct choice.
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