Al2O3 was leached with alkali to get X. The solution of X on passing of gas Y, forms Z. X, Y and Z respectively are

Strategy: Trace the leaching of Al₂O₃ with alkali and subsequent reaction with CO₂ to identify X, Y, Z.

Step 1: Leaching Al₂O₃ with Alkali (X) $\ce{Al2O3 + 2NaOH + 3H2O -> 2Na[Al(OH)4]}$ So X = $\ce{Na[Al(OH)4]}$ (sodium tetrahydroxoaluminate).

Step 2: Gas Y reacts with X to form Z When $\ce{CO2}$ (gas Y) is passed through the X solution, it precipitates hydrated aluminium oxide: $\ce{Na[Al(OH)4] + CO2 -> Al(OH)3 \cdot xH2O (Z) + NaHCO3}$ or more formally, the precipitate is $\ce{Al2O3 \cdot xH2O}$ (hydrated alumina / gelatinous aluminium hydroxide).

So Y = $\ce{CO2}$, Z = $\ce{Al2O3 \cdot xH2O}$

$\boxed{\text{Answer: (D) }X = \ce{Na[Al(OH)4]},\ Y = \ce{CO2},\ Z = \ce{Al2O3 \cdot xH2O}}$