A pronoun can substitute a noun or a noun phrase. Among the options above only C. him and D. They are pronouns: stories is a noun and many could be a pronoun if is was standing alone, but here it is a part of a noun phrase.
C. him is singular, so the right answer is D. They
It is true that the synthesis of a large collection of information that encompasses well-tested hypotheses about certain aspects of the natural world is known as a scientific theory.
