.
It is between the sum 8+12 = 20 ft and the difference 12-8 = 4 ft of the lengths of the two given sides,
excluding the extreme values of 4 ft and 20 ft.
When I say "the difference", I mean the POSITIVE difference, of course.
Options A) and C) do not work - they are not possible.
Other optional lengths from the list are possible.
Actually, there are INFINITELY MANY possible values for the length of the third side.