A simple subject is whom or what the subject is about.
The answer is noun.
Ok it would be C. (Park's) because when you put 's at the end of a word it means it belongs to something and in this sentence the gates belong to the park.
I would say the correct answer is that has an undetermined answer.
If a subject has been thoroughly covered, then there is no need for this researcher to do it all over again. If it's impossible to answer, there is no need to try. If it can be answered easily, then it doesn't really require a research. This leaves us with the last option, because a researcher might want to try and determine the answer.