Step 1: Identify the reaction type.
The reaction involves addition of a Grignard reagent to an aldehyde. Step 2: Analyze benzaldehyde reaction.
Benzaldehyde reacts with methyl magnesium bromide to form a secondary alcohol after hydrolysis. Step 3: Product formation.
\[
\mathrm{C_6H_5CHO + CH_3MgBr \rightarrow C_6H_5CH(OH)CH_3}
\] Step 4: Conclusion.
Therefore, \(\mathrm{CH_3MgBr}\) reacts with benzaldehyde to give 1-phenylethanol.