Question

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

A. tuna spawned

B. for a long time

C. where tuna spawned

D. no one knew where

Answer 1

C. where tuna spawned

without this part of the sentence, it wouldn't make any sense.
no one knew what? where tuna spawned.

Answer 2

Yes. C. noun clause (has subject and verb) indicating what was not known