Which words in the sentence are the complete noun clause? for a long time, no one knew where tuna spawned. a. no one knew where b. tuna spawned c. where tuna spawned d. for a long time.

I would say the answer is C. Actually am not a native english speaker but I think its the answer.

The answer is C, where tuna spawned. A noun clause is a dependent clause that essentially fills the role of a clause. Since a noun clause is dependent, it does not form a complete thought or full sentence when pulled from the master sentence.