Which words in this sentence are an independent clause?

Scientists have found traces of an ancient forest that once grew on an empty lot in New Jersey when the region had a tropical climate.

Awhen the region had a tropical climate
BScientists have found traces of an ancient forest
Cthat once grew on an empty lot in New Jersey
Don an empty lot in New Jersey

Verified Answer :

The correct answer is B. An independent clause is a sentence that can exist in that form on its own. The remaining options cannot exist in this form, as they need to be completed, they are dependent on the main clause. Independent clause is finished, it doesn't need additional elements.

just sayin' i dont know why he's verified because i just took the test and he's wrong.. :(