Illusion of choice.
The sum of the digits of any multiple of 9 equal to or less than 90 is 9 (every recall our times tables from way back in 2nd grade). Subtracting 5 from 9 results in 4. 4 corresponds to D.
If you got something other than D, you are REALLY bad at math.
List of Countries whose names begin with D (in English):
Easy to assume Denmark from that point.