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We know (from the fact that it's a monoprotic acid) that the acid + base reacts in a 1:1 ratio in this neutralisation reaction, therefore if we look at moles in each condition:
X: all salicylic acid used up
Y: moles cancel out
Z: salicylic acid left in solution
Z is the only one with salicylic acid left in solution, and therefore HA that can form a buffer solution.