I think it's b (7.5ft.)

this is how I got there:

first we know the d=20, therefore r=10

the lane is supposed to be 12ft wide with equal distance from the left and right side, so this gives us a distance of 6ft from O in both directions.

Now we are supposed to find out how "tall" vehicles may be at the most.

This is the point I labelled "max. height" on the upper right side.

This now gives us a triangle with the base of 6ft. (half the lane) and a hypothenuse of 10ft. (it's the radius)

This is a 3-4-5 triangle with a factor of 2.
10=5*2
6=3*2

x=4*2=8

Now the maximum height of the vehicle should be 1/2foot smaller. Thus: 8ft.-1/2ft = 7.5ft.

This leaves us with answer b.

Can you please give OA?