Your answer should be WHO.
Think of it like this: You wouldn't say "The boy it (a variant of 'which')", because it simply just sounds odd. Instead, you're wired to think "The boy, he (an unknown name, so it would be 'who')".
Answer:
Because few people have ever seen it, The Giant Squid is one of the great sea monster mysteries of the deep sea.
Explanation:
i believe it is this one.
A source that is a trustworthy authority on a subject is a credible source.
