D. 54° as you know an external angle has to equal up to 360
when you start plugging in 360 by 36 gives you 10
360 by 40 gives you 9
360 by 45 gives you 8
360 by 54 gives you 6.666 repeating you know that it isn't possible
you cannot have a repeating number of sides as an angle measure
that's how you get that answer
