The answer is C,
Coterminal when involving angles is an angle that is 360 degrees greater or lesser. In order to find whether something is coterminal or not you take one of your angles and subtract or add 360 from it until you reach that number. If you add/sub 360 and you can't ever equal the other number than they are not coterminal.
Example:
591 - 360 = 231
So that means
231 degrees = 591 degrees
Example 2:
51 - 360 = 309
51 + 360 = 411
411 + 360 = 771
771, 411, and 309 /= 591 or 231
So 51 degrees /= 231 or 591
C: 51 is not coterminal!
Hope this helped!