The answer is A. This sentence capitalizes the first letter of the first word in the sentence, which is correct. It also has Saturday capitalized, and as Saturday is considered a proper noun, this is also correct. London, being a city, is another proper noun, therefore, it is necessary to have it capitalized. Sentence B can be identified as incorrect solely because 'afternoon' is capitalized. Afternoon is a noun, but not proper. Choice C makes the same mistake, as well as others. Finally, D capitalizes 'city,' which is not a proper noun. A proper noun is defined as a name that is used for a person, place, or organization.