Which of the following book titles is correctly capitalized?

The title "To Kill a Mockingbird" is correctly capitalized according to standard title capitalization rules, where the first and last words of the title, as well as all major words in between, should be capitalized. In this case, "To," "Kill," "Mockingbird," and "a" are all at the beginning of the title or are important words, making their capitalization necessary and correct.

In contrast, the other options do not follow these capitalization conventions. "The Lion, The Witch, and The Wardrobe" incorrectly capitalizes "The" in the middle of the title when it does not need to be capitalized. "A tale of two cities" does not capitalize "tale," which should be capitalized as it is a significant word in the title. Finally, "Computers for dummies" improperly keeps "dummies" in lowercase; in formal title capitalization, it should be "Dummies."

Thus, "To Kill a Mockingbird" stands out as the only title that adheres to the correct rules of capitalization for titles.