Reaction type

Dehydration of an alcohol with concentrated $H_2SO_4$ is an acid-catalyzed elimination reaction that gives an alkene.

What happens first?

Alcohol contains the $-OH$ group, but $OH^-$ is a poor leaving group. So, before elimination can occur, the alcohol must be converted into a better leaving group.

Initiation step

In acidic medium, the lone pair on oxygen of the alcohol attacks a proton from the acid:

$R-OH + H^+ \rightarrow R-OH_2^+$

This is protonation of the alcohol molecule.

Why not the other options?

• A: formation of carbocation — this may occur later (especially in $2^\circ$ and $3^\circ$ alcohols), but it is not the first step.
• B: elimination of water — water leaves after protonation, so this is also not the initiation step.
• C: formation of an ester — not the initiating step in alkene formation by dehydration.
• D: protonation of alcohol molecule — correct, because this is the first event that activates the alcohol.

Conclusion

The initiation step is:

$\boxed{\text{Protonation of alcohol molecule}}$