Assertion : MeMgBr should be prepared under perfectly anhydrous conditions.
Reason: Grignard reagent reacts with water.
A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
C. Assertion is correct but Reason is incorrect
D. Assertion is incorrect but Reason is correct

Hint: Grignard reagent is used for the preparation of alkanes. It is an organometallic compound that contains carbon-metal bonds. The reactivity of these compounds increase with the percent ionic character of the carbon-metal compound. they react explosively with water and bursts into flame on exposure to air.

Complete step by step answer:
These reactions are carried out in ether solvents as organometallic compounds are strong bases care must be taken to exclude the moisture. Grignard reagent should be prepared under anhydrous conditions, because it is very reactive. It reacts very quickly with any source of proton to give hydrocarbon. Therefore, it is necessary to avoid moisture from Grignard reagents. Let us understand this with the help of a reaction:
$
  RX + Mg\xrightarrow{{dryether}}{R^ - }M{g^ + }X \\
  {R^ - }M{g^ + }X + {H_2}O \to RH + X - Mg - OH \\
 $
MeMgBr should be prepared under perfectly anhydrous conditions. Grignard reagent reacts with water to form alkane and magnesium hydroxy halide. Here Both Assertion and Reason are correct and Reason is the correct explanation for Assertion. Under the following reaction.
$MeBr + Mg\xrightarrow{{E{t_2}O}}MeMgBr$
Alkyl magnesium halide (Grignard reagent)
Grignard reagents have great use in organic synthesis. Not only Grignard reagents are strong bases, they are also powerful nucleophiles and since water is a stronger acid as compared to Grignard reagent it removes its proton. This is also an acid-base reaction.

Note:
A Grignard reagent has a formula RMgX where X is a halogen and R is an alkyl or aryl group. When it reacts with water to produce alkanes. Grignard reagents are sensitive to acidic compounds which is why it is prepared in anhydrous conditions.