Decreasing order of dehydration of the following alcohols is:

Step 1: Factors governing ease of dehydration

Dehydration of alcohols involves loss of water to form an alkene. The rate depends on:

1. Stability of the carbocation formed (for E1 mechanism) or ease of H abstraction (E2).
2. More stable carbocation → faster dehydration.
3. Order of carbocation stability: benzylic > tertiary > secondary > primary.

Step 2: Assign types

• (A) Phenol: The C-O bond in phenol has partial double bond character (resonance). Phenol is very resistant to dehydration — the phenoxide C-O is essentially unsusceptible to typical E1/E2 conditions. Slowest/Hardest to dehydrate.

• (B) Benzylic secondary alcohol (e.g., 1-phenylethanol): Forms stable benzylic carbocation → very easy to dehydrate. Fastest.

• (D) Tertiary or highly branched alcohol: Forms tertiary carbocation → relatively easy. Second fastest.

• (C) Secondary aliphatic alcohol (cyclohexanol): Forms secondary carbocation → moderate. Third.

Order: B > D > C > A

Key Points to Remember:

• Phenol is the hardest to "dehydrate" in a normal sense — its C-O bond is strengthened by resonance.
• Benzylic alcohols dehydrate most easily (form stable benzylic carbocations).
• General dehydration ease: benzylic > 3° > 2° > 1° >> phenol.