[AManWithNoPlan]

This is the first stoichiometry problem where I had to add the compounds in the first portion to get the mass of a compound in the second. The answer is D.

[Vidya]

what you have done is not clear?
Its a problem of  limiting reactant
so go from  g of Mg  to g of MgCl2
write all units in dimensional analysis ..I can see you are doing error in plugging the ratio ...
then go from g FeCl3 to g of MgCl2
In your calculations shown in attached file you have combined both the steps...these are two separate steps

[docnet]

You need to develop a system that allows you to meticulously keep track of units and your progress throughout the calculations. Like with dimensional analysis problems, I thought these "limiting reactant" type problems as well as "combustion analysis" type problems (where there are multiple elements to keep track of) it helps to note additional information by each number, for example:

39g _{FeCl3 reactant}

or 

.29g _{MgCl2 sample}

And so on. It helps me keep track of where I am in the calculation process, if I get lost.

[Babcock_Hall]

I have not looked over your calculation in detail, but a common mistake is not to take correct account of the stoichiometric coefficients.