Он a. b. но он c.

Grignard reagent: Alkyl magnesium halide is called as Grignard reagent( R-MgX ).

Grignard Reagent is prepared by the reaction of alkyl halide with magnesium in dry ether.

$\mathrm{R}-\mathrm{X} + \mathrm{Mg} \stackrel{\mathrm{dryether}}{\to } \mathrm{R}-\mathrm{MgX}$

Here, R is alkyl, alkenyl and aryl group and X is halogen Cl, Br, I.

Two equivalents of Grignard reagents react with ester to form tertiary alcohol.

$2\mathrm{equiv}.\mathrm{R}-\mathrm{MgX} + {\mathrm{RCOOR}}^{\text{'}}\to {\mathrm{R}}_3\mathrm{C}-\mathrm{OH}$