The difficulty of replacing Cl^– depends on the stability of the carbocation formed after Cl^– leaves. More stable carbocations form more easily, making substitution easier. The most difficult replacement occurs when the carbocation is least stable.

Among the options, the structure that forms a primary carbocation (least stable) will have the most difficult replacement. The structure with the Cl attached to a simple alkyl chain (no resonance or tertiary carbon) is the answer.

Final Answer: The first option (simple alkyl chloride) has the most difficult replacement.