Hello! The answer to your question would be as followed:
The only time you can leave your child unattended in the car is B. Only if they are older than six-years-old
A. is incorrect because this can apply to all ages, doesn't matter how long your gone, a child under the age of six should not be in a car alone.
C. is incorrect because yes although the windows are rolled down to not cause heat stress, many more problems can occur by leaving a child in the open to other people.
D. is incorrect because, well it does not make sense at all