Just look at the mass of the compound.
If all Cl has reacted, you should get MgCl2 and the mass is
incorrect.
Likewise for b which will get you MgBr2
Option C is just like A meaning all the Cl has reacted since
halogen mass of 160g is roughly the mass of 1 mole of Br2.
Option D is the only option that fits the mass of the compound
obtained.
Ok. So the mass, (referring to option A), do you mean the mass of
139.5 for MgCl2 is incorrect? Likewise for option B(you are
referring to MgBr2?)
And for option C, if the mass of halogen left is 160g, like what
you said, all the chlorine will have reacted away since 80 + 80 =
160 (gives us the Mr of Br2).
Just paraphrasing to see if i got the correct idea. Thanks!