Benzyl magnesium bromide is a Grignard reagent. The carbon atom attached to metal ' $\mathrm{Mg}$ ' is strongly basic and abstracts $\mathrm{D}^{+}$from heavy water. The reaction of a Grignard reagent with $\mathrm{D}_2 \mathrm{O}$ ("heavy water") provides a convenient method for introducing a deuterium atom into a molecule at a specific location.
