Problem: a circular mat with diameter 18 inches is placed on a square tabletop , each of whose sides is 24 inches long . which of the following is closest to the fraction of the tabletop covered by the mat ? 
Options: ['a ) 5 / 12', 'b ) 3 / 5', 'c ) 1 / 2', 'd ) 3 / 8', 'e ) 5 / 6']
d