I am no tutor but I think I can help you with this question. The passage states:
“A history book written hundreds of years ago contains several inconsistencies. Some scholars argue that because the book contains inconsistencies, the author must have been getting information from more than one source.”
C says, “the history book’s author used no source that contained inconsistencies related in the history book.”
If you were to negate that sentence, “the history book’s author used a source that contained inconsistencies related in the history book” the whole argument falls apart. If the author used only one (1) source that had inconsistencies, then the author wouldn’t need to confer with other sources making the conclusion, “the author must have been getting information from more than one source” not true. Because it strengthens and the negation leads to the conclusion not being true, C is the correct answer!