Answer:The correct answer is option C.
Explanation:

At STP, 1 mol of gas occupies 22.4 L of volume
So, 100.0 L volume of
will be occupied by :

According to reaction 2 moles
are obtained from 1 mole of
.
Then,4.4642 moles of
will be obtained from :
that is 2.2321 moles of 
Mass of
gas:
Moles of
gas Molar mass of
gas:

Hence, the correct answer is option C.
Since the beaker was heated we can asume that only magnesium cloride is left in the beaker, therfore the difference between the beaker with magnesium chloride and the empty beaker give the mass of magnisium chloride:
The closeness of a measurement to its true value is a measure of its Accuracy.