The answer is 2.0 moles.
From their coefficients in the balanced chemical equation below for the reaction of tin dioxide with hydrogen gas to produce tin and water:
SnO2(s) + 2H2(g) ==>Sn(l) + 2H2O(g)
two moles of hydrogen gas is to produce one mole of tin. We use this mole ratio to compute for the number of moles of tin as:
moles of Sn = 4.0mol H2 (1mol Sn/2mol H2) = 2.0 mol
