Grignard's reagent is prepared by the action of magnesium metal on:

Options

A. Alcohol
B. Phenol
C. Alkyl halide
D. Benzene

Explanation

To solve the question regarding the preparation of Grignard's reagent, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Grignard's Reagent**:
Grignard's reagent is a type of organomagnesium compound, represented by the general formula R-Mg-X, where R is an alkyl or aryl group, Mg is magnesium, and X is a halogen (such as Cl, Br, or I).

2. **Identifying the Reactants**:
The question asks about the action of magnesium metal. Grignard's reagent is specifically formed by the reaction of magnesium with alkyl halides.

3. **Reaction Conditions**:
The reaction typically occurs in the presence of dry ether, which acts as a solvent. This is crucial because Grignard reagents are highly reactive and can react with water or moisture.

4. **The Reaction**:
The general reaction can be represented as:
\[ R-X + Mg \xrightarrow{\text{dry ether}} R-Mg-X \]
where R-X is the alkyl halide.

5. **Example**:
For instance, if we take ethyl chloride (C2H5Cl) as the alkyl halide, the reaction with magnesium in dry ether would yield:
\[ C2H5Cl + Mg \xrightarrow{\text{dry ether}} C2H5-Mg-Cl \]

6. **Conclusion**:
Therefore, Grignard's reagent is prepared by the action of magnesium metal on alkyl halides.

### Final Answer:
Grignard's reagent is prepared by the action of magnesium metal on **alkyl halides**.

---