In this reaction, Zn(s) + 2 HCl(aq) → ZnCl2(aq) + H2(g), what is the substance oxidized?

The Zinc is oxidized and the Hydrogen reduced. The way to determine this is to identify where the elements are in the reaction. If an element decomposes, it is oxidized. The Zinc turns from pure metal to a salt. If an element is created, it is reduced. The Hydrogen becomes Hydrogen gas.