Which relation is a function?

Only the one on the bottom left, the one that looks like the absolute value function, is a function.  The others fail the vertical line test; to be a function every vertical line must intersect the curve at at most a single point.

bottom left

i leave the calculator work to you.

7. diameter 4 miles, circumference 4π miles

8. radius 0.2 miles, circumference 0.4π miles

[this is a bad problem because the actual radius of a car on the track is less than 0.2 miles so the answer is less than 0.4π miles]

9. radius 6 feet, circumference 12π feet

10. diameter 14 cm, circumference 14π cm

11. diameter 4.8 feet, circumference 4.8π feet