molarity = number of moles of solute / liters of solution
Therefore,
number of moles = molarity * volume
number of moles of HF = 2.5 * 3.8 = 9.5 moles
From the balanced equation given:
one mole of Mg is required to react with two moles of HF, therefore, to get the number of moles of Mg that reacts with 9.5 moles of HF, we will just do a cross multiplication as follows:
number of moles of Mg = (9.5*1) / 2 = 4.75 moles
From the periodic table:
molar mass of Mg = 24.305 grams
number of moles = mass / molar mass
therefore,
mass = number of moles * molar mass
mass og Mg = 4.75 * 24.305 = 115.44 grams
Answer: (c)