A hydrocarbon X undergoes the reaction under suitable conditions as shown below:

Y only

Reason — The reaction shown is cracking (thermal decomposition) of butane (X = C₄H₁₀), a saturated hydrocarbon, into propene (Y = C₃H₆) and methane (Z = CH₄).

• X (butane, C₄H₁₀) — saturated (only single bonds).
• Y (propene, C₃H₆) — unsaturated (contains one C=C double bond).
• Z (methane, CH₄) — saturated (only single bonds).

Hence, only Y is unsaturated.