Br2 in light will go to the most stable radical (tertiary). Then Mg and ether will make Grignard (MgBr). Grignard is a carbanion and also a very strong base. So by adding D2O, carbanion will receive D.
The answer is B.
D is Deuterium, it is an isotope, or the heavier form of Hydrogen. When you add water to Grignard, carbanion gets protonated, meaning when yo react it with water you add an H to the carbanon.
Here, instead of adding an H to the carbanon, we add D.
Hope this helps.