Le Chatelier's principle for exothermic equilibria

Steps:

• Lower yield of Z at higher temperature means increasing T shifts equilibrium left, decreasing products.
• Le Chatelier's principle states higher temperature favors the endothermic direction.
• Thus, reverse reaction is endothermic, so forward reaction (forming Z) is exothermic with ΔH < 0.
• Option C has ΔH = -100 kJ mol^{-1} and all gaseous reactants/products.

Why C is correct:

• Negative ΔH indicates exothermic forward reaction; per Le Chatelier's principle, higher temperature shifts equilibrium left, reducing Z yield.

Why the others are wrong:

• A: Positive ΔH (endothermic forward); higher temperature increases Z yield.
• B: Negative ΔH, but Z(l) implies phase change; temperature effect on liquid-gas equilibrium not purely ΔH-driven without more info.
• D: Positive ΔH and Z(l); higher temperature would increase Z yield.

Final answer: C