Final answer:
The correct answer is A. Realized, as it fits the past tense narrative of the sentence.
Step-by-step explanation:
Carrie waited around for a while; it was only fifteen minutes later that she realized that she had missed the bus. The correct answer here is A. Realized. The past tense 'realized' fits the context of the sentence, indicating that Carrie came to a conclusion or became aware of something after the fact. The sentence is describing a past event, which requires the use of the past tense verb.