The British attempted to break the land-speed record and the sound barrier using a jet-powered car. They made their attempt in the morning because this is when the air was coolest. This suggests that the relationship in which graph is true?

Your answer is B.

The relationship between medium temperature and speed of sound is a direct relationship: when one factor increases, the other increases as is shown in graph B. The British would choose the the time of day which would give the lowest speed of sound, because this would be easiest to break. Graph B shows that the lowest speed of sound would occur with the lowest air temperature - in the morning.