The correct option is A This summer, we are traveling to Munich and Zurich.
Capital letters are used for the first letter of the first word of a sentence and for proper nouns. In other words, capitalize the names of people, specific places, and things. For example, We don't capitalize the word "bridge" unless it starts a sentence, but we must capitalize Brooklyn Bridge because it is the name of a specific bridge. Here, 'Munich' and 'Zurich' are proper nouns, so must be capitalized. 'This' is the first word at the start of the sentence, so must also be capitalized. Hence, the correct option is A). Summer is not a proper noun, so it need not be capitalized.