Cyclohexene is best prepared from cyclohexanol by which of the following reagents? 

conc. H₃PO₄

The correct answer is option1. \(conc. H_3PO_4\)_.

The preference of phosphoric acid \((H_3PO_4)\) in suppressing substitution and favoring elimination during cyclohexanol dehydration is accurate and well-written. Here's a breakdown of the points:

1. Nucleophilic counterions: Chloride \((Cl^-)\) and bromide \((Br^-)\) ions, present in \(HCl\), \(HCl/ZnCl_2\), and \(HBr\), act as nucleophiles.

2. Competing reactions: These nucleophilic counterions can participate in side reactions with the carbocation intermediate formed during the dehydration process, leading to substitution products like cyclohexyl chloride or bromide.

3. Non-nucleophilic counterion: Phosphate \((PO_4^{3-})\) ion in \(H_3PO_4\) is a bulky and poorly nucleophilic species.

4. Minimized side reactions: Due to its low nucleophilicity, \(PO_4^{3-}\) has minimal interaction with the carbocation, minimizing the possibility of substitution and promoting the desired elimination reaction to form cyclohexene.

Therefore, choosing a reagent with a non-nucleophilic counterion, like \(H_3PO_4\), significantly increases the selectivity towards the elimination pathway, yielding cyclohexene as the primary product. This strategy is crucial for optimizing the synthesis of cyclohexene from cyclohexanol.