The Grignard reagent can be prepared by using magnesium in dry ether on

$C_2H_5Br$

The correct answer is Option (2) → $C_2H_5Br$

Grignard reagents are prepared by reacting alkyl or aryl halides with magnesium metal in the presence of dry ether.

General Reaction:

$R-X + Mg \longrightarrow R-Mg-X$ (in dry ether)

Detailed Explanation of Options

Option 1: $C_2H_5OH$ (Ethanol) — Ethanol contains an acidic hydrogen attached to oxygen. Instead of forming a Grignard reagent, it reacts with magnesium to produce ethoxide or destroys any formed Grignard reagent via proton donation.

Option 2: $C_2H_5Br$ (Ethyl bromide) — As an alkyl halide, it reacts readily with magnesium in dry ether to form ethyl magnesium bromide, which is a Grignard reagent.

$C_2H_5Br + Mg \rightarrow C_2H_5MgBr$

Option 3: $C_2H_6$ (Ethane) — Ethane is an alkane and lacks a halogen atom. Since Grignard reagents require alkyl halides, ethane cannot react with magnesium to form one.

Option 4: $C_2H_5NO_2$ (Nitroethane) — Nitro compounds do not form Grignard reagents; instead, they interfere with or react with the reagent due to their reactive functional group.