Dehydration of alcohols to alkenes:
Dehydration of alcohols is a common reaction used to convert alcohols into alkenes. This reaction involves the removal of a molecule of water from the alcohol molecule.

The reaction mechanism:
The reaction proceeds via an acid-catalyzed mechanism, where the alcohol molecule is protonated by a strong acid such as concentrated sulfuric acid (H2SO4) to form an oxonium ion. This oxonium ion is highly unstable and loses a molecule of water to form a carbocation intermediate. The carbocation intermediate is then deprotonated by a base (such as a neighboring water molecule or the conjugate base of the acid catalyst) to form the alkene product.

The initiation step:
In the initiation step of the dehydration reaction, the alcohol molecule is protonated by the acid catalyst (H2SO4). This protonation occurs on the oxygen atom of the alcohol, resulting in the formation of an oxonium ion. This oxonium ion is highly reactive and serves as an intermediate in the reaction.

Why is option 'D' correct?
Option 'D' states that the initiation step of the reaction is the protonation of the alcohol molecule. This is indeed the correct answer. Protonation of the alcohol molecule is the first step in the reaction and leads to the formation of the oxonium ion intermediate.

The protonation of the alcohol molecule occurs because the oxygen atom of the alcohol is a nucleophile and can react with the acid catalyst (H2SO4). The acid catalyst donates a proton to the oxygen atom, resulting in the formation of the oxonium ion. This protonation step is essential for the subsequent loss of water and the formation of the carbocation intermediate.

Conclusion:
In the dehydration of alcohols to alkenes, the initiation step involves the protonation of the alcohol molecule by a strong acid catalyst. This protonation leads to the formation of an oxonium ion, which serves as an intermediate in the reaction. Option 'D' correctly identifies the initiation step of the reaction.