Final answer:
The best option to complete the sentence is (C) 'as its interest in nonfiction books' because it appropriately compares the public's increasing interest in documentary films to its interest in nonfiction books.
Step-by-step explanation:
The correct option : c
This choice correctly compares the public's appetite for one type of medium to its appetite for another. The public's increasing interest in documentary films is likened to its growing interest in nonfiction books, making option C the best choice. The phrase 'as its interest' properly draws a parallel between the public's attitudes towards both documentaries and nonfiction books, which are different forms of media, but both present factual narratives.
Other options like (A) 'like nonfiction books' and (B) 'as nonfiction books' incorrectly suggest that documentary films are comparable to nonfiction books themselves rather than the public's interest in them. Option (D) 'like their interest in nonfiction books' improperly uses 'their' which could imply multiple entities, whereas 'its' correctly refers back to the singular noun 'public.' Likewise, option (E) 'like its interest in nonfiction books' uses 'like' when 'as' is more appropriate for drawing a comparison of the nature of the public's interest in both forms of media.