The first step to answering this item, is to balance first the chemical reaction. This is shown below.
Mg(s) + 2HCl (aq) --> MgCl₂(aq) + H₂(g)
HCl is multiplied by 2 to equalize the number of the hydrogen and chlorine.
The molar masses of magnesium and hydrogen gas are 24.3 g and 2 g, respectively. Using dimensional analysis and conversions, mass of hydrogen is calculated through the equation,
Mass of H₂ = (0.373 g Mg)(1 mol Mg/24.3 g Mg)(1 mol H₂/ 1 mol Mg)(2 g H₂/1 mol H₂)
Mass of hydrogen = 0.031 g of hydrogen
ANSWER: 0.031 g H₂