First, you find the simple subject and simple predicate.
If the '-ing' form of the verb is not part of the simple predicate, then determine how it is used in the sentence. If it's in a noun position, then it's a gerund. If it is used as an adjective, then it will be a participle.
Answer:
Documentary
Explanation:
Encyclopedia is Tertiary, Diary is Primary, and Interview is Primary. Documentary comes directly from the main source, the person with the camera. Nothing is in the middle.
