Which words in the sentence are the complete noun clause?
My younger brother complains about whatever is on his plate.
A. Whatever is on his plate
B. My younger brother
C. On his plate
D. My younger brother complains about whatever
noun clauses or dependent clauses are the part(s) of a sentence that cannot stand on their own as a complete thought, so in this sentence it would be C. On his plate because it does not sound like a complete thought on its own.