Acetyl bromide reacts with excess of ${\mathrm{CH}}_3\mathrm{MgI}$ followed by treatment with a saturated solution of ${\mathrm{NH}}_4\mathrm{Cl}$gives

The reaction is

The tertiary alcohol produced when acetyl bromide combines with excess ${\mathrm{CH}}_3 \mathrm{Mgl}$I and then is treated with a saturated solution of ${\mathrm{NH}}_4\mathrm{Cl}$ is 2-methyl-2-propanol, according to the supplied option.