Substance X reacts with dilute HCl violently, Y reacts slowly while Z does not react at all. X, Y and Z could be respectively.

K (X) reacts with HCl violently

Al (Y) reacts slowly with HCl and

Cu (Z) does not react with HCl

Hence X, Y and Z are K, Al and Cu.

$\therefore$ So the order of reactivity of X, Y and Z metals is $X>Y>Z$.