The acid-catalyzed dehydration of alcohols to alkenes proceeds via a mechanism involving the formation of a carbocation intermediate. The rate of the reaction is directly proportional to the stability of the carbocation formed in the rate-determining step.

1. Carbocation Stability: The general order of carbocation stability is Tertiary ($3^{\circ}$) > Secondary ($2^{\circ}$) > Primary ($1^{\circ}$). Electron-donating groups (+I effect) and resonance increase stability, while electron-withdrawing groups decrease it.
2. Reactivity Order: Consequently, tertiary alcohols are the most reactive towards dehydration, followed by secondary, and then primary alcohols.

Assuming the standard set of compounds for this common NEET question (where (b) is a primary alcohol like 1-propanol, (a) is a secondary alcohol like 2-butanol, (c) is a secondary alcohol with specific features, and (d) is a tertiary alcohol like 2-methylpropan-2-ol), the order of reactivity increases from primary to tertiary: (b) < (a) < (c) < (d).