Question

In the given reaction product P is,

Answer 1

Here HF is a reagent. A reagent can be a substance or mixture of compounds which are used in chemical analysis or reactions. When a reagent is added to a system it causes chemical reactions. For a particular reaction, a particular reagent gets consumed in the process of the chemical reaction.

Complete step by step answer:
In presence of HF, it acts as an acid or proton donor. And benzene acts as a nucleophile,
The reaction undergoes by the formation of carbocation as follows,

Additional information:
To convert benzene to ethylbenzene there is a very well-known reaction which is the Friedel-Craft reaction. It is a coupling reaction between aromatic compounds (benzene) and alkyl halide or acyl halide in presence of anhydrous Lewis acid $AlC{l_3}$. In presence of aq. $AlC{l_3}$ it reduces the nucleophilicity of the benzene ring by the formation of the cation of the benzene ring. the reaction is shown below,

Therefore, the reagent used for converting benzene to ethylbenzene is: ${C_2}{H_5}CI$, anhydrous $AlC{l_3}$
Friedel-Craft reaction with aryl halide is not possible, because the lone pair of halogen undergoes conjugation with the benzene ring and forms a partial double bond. Due to this reason, only alkyl halide can be used as a halide component in a Friedel-craft reaction.

Note: Benzene is an aromatic compound with the formula ${C_6}{H_6}$. When one or more than one hydrogens of benzene are replaced by any groups then several compounds are formed. Now for the IUPAC nomenclature at first, we prefix the name of a substituent to the word benzene. For example, chlorobenzene, methylbenzene, etc.