Typically, Iron reacts with HCl to produce Iron (III) Chloride and hydrogen gas.
Fe + 2HCl = FeCl2 + H2 (g)
5.6g = 2.016×5.6/(55.85) = 0.20g (approx.)
Not only iron, any other metal placed higher up (with respect to hydrogen) in the activity series releases hydrogen gas from HCl through a similar reaction. But metals placed below hydrogen do not produce hydrogen on reaction with HCl (or other mineral acids). Here, the amount of hydrogen produced in the reaction between impure iron and HCl is slightly more (0.21g) than what was expected of a pure piece of iron metal (0.20g). Hence the impurity must be a metal placed higher in the activity series. Moreover, the metal must be lighter than iron so that the same mass contains more number of atoms, which in turn produces more number of molecules of hydrogen gas. Among the given options copper is placed lower than hydrogen, hence omitted. The remaining three metals are all placed higher than hydrogen, but atomic mass of Zn and Pb are higher than iron, hence omitted. The only metal fulfilling all the required criteria, therefore, is Mg. Hence the answer is (b) Mg.