The answer to your question would be that the sentence that conforms to the style guidelines is the following one: I bought three items at the store: tissues, detergent, and crackers. That is, your answer would be A.
A colon means "that is to say" or "here's what I mean". You should use a colon to introduce a series of items and you should not capitalize the first item after the colon. In the selected sentence, the series of items specifies the three items at the store.
