In any number system the digits cann't be greater than its Radix
Eg; If we consider Radix-2 that is Binary number system we can have 0-1 but not 2
Same way in Radix-3 all the numbers should be less than 3 i,e; 0,1,2 but 3 is not possible
This another blunder from ACE tests i guess
X->YZ , Y->XZ , ...