The reactivity of the C-X bond towards nucleophilic substitution depends largely on the stability of the carbocation intermediate formed ($S_N1$ mechanism) or the steric hindrance ($S_N2$). Compound III is a tertiary alkyl halide ($(\text{CH}_3)_3\text{C-X}$), which readily undergoes heterolytic cleavage to form a highly stable tertiary carbocation, making it very reactive towards nucleophiles. Compound IV is a secondary alkyl halide ($(\text{CH}_3)_2\text{CH-X}$), which forms a less stable secondary carbocation and is therefore less reactive than III. Although the structures for I and II are missing from the provided text (in standard literature, these correspond to aryl and vinyl halides, which are highly unreactive due to partial double bond character arising from resonance), the relative order must conclude with IV < III. Option A (I < II < IV < III) correctly reflects this relative reactivity.