This is a good sentence. However, if it's an essay then try to be less vague. Add some details about the subject. Like why are they a resource?
Answer:
reliable (or trustworthy)
Explanation:
"Credible" means "able to be relied upon or trusted".
Since no comma is used after the word "had" so option C is the right answer
integral would mean important, for if something is a "whole" of something, then it must be vital.
Answer: i’m pretty sure it’s c