During dehydration of alcohols to alkenes by heating with conc. `H_(2) SO_(4)` the initiation step is :

Explanation

Under the acidic conditions, alcohol is initially protonated and then oses `H_(2)O` to form a carbocation. If `HCl` is used, then `Cl^(-)` ion being a strong nucleophile will result in nucleophilic substitution to form alkyl chloride. However, `HSO_(4)^(-)` ion released by `H_(2)SO_(4)` is a very weak nucleophile since the negative charge is dispersed on four electronegative oxygen atoms. It cannot participate in the nucleophilic substitution. It will rather act as a base and eliminate a proton to form alkene as the product as follows :

Concentrated `HNO_(3)` is a powerful oxidising agent. It will cause oxidation of alcohol to aldehyde and then to acid. thus, out of the mineral acids listed, dehydration is carried by concentrated `H_(2)SO_(4)`. even phosphoric acid can be used.