John didn't sleep well because A summer was ending. B he was excited about going to school. C he heard heavy traffic and car horns. D his sister Audrey kept him awake.

John didn't sleep well because C. he heard heavy traffic and car horns.

Explanation:

John and Audrey are characters in the story "The First Day of School", by R. V. Cassil. The story begins with John waiting for his sister Audrey as the time for the them to go to school gets closer. Audrey has slept all night, but John hasn't. He could hear heavy traffic at midnight, so he wasn't able to sleep at all after that:

She must have slept all right - and for this John felt both envy and a faint resentment. He had not really slept since midnight. The heavy traffic in town, the long wail of horns as somebody raced in on the U. S. highway holding the horn button down, and the restless murmur, like the sound of a celebration down in the courthouse square, had kept him awake after that. Each time a car had passed their house his breath had gone tight and sluggish. It was better to stay awake and ready, he had told himself, than to be caught asleep.

It turns out that John and Audrey are African Americans. Their first day of school is filled with tension and anxiety, since they and a few others will be the first ones to attend a school for white people in his town. The National Guard has been called to protect them from all the people who have gathered to jeer and boo - and maybe even attack - them as they make their way to school.