Final answer:
The correct use of an apostrophe in the given sentences is 'C. The magazine's back cover was filled with quotes from famous people,' which correctly shows possession for a singular noun.
Step-by-step explanation:
The sentence that uses an apostrophe correctly is 'C. The magazine's back cover was filled with quotes from famous people.' This sentence employs the apostrophe to show possession, indicating that the back cover belongs to the magazine. The apostrophe is used along with an s for singular nouns to show possession, as in 'the owner's insurance' or 'the waitress's coat.' When the noun is plural and ends in -s, the apostrophe is placed after the final -s, such as 'households' incomes.' It's also important to note that apostrophes are not used to make nouns plural.