Magnesium reacts with hydrochloric acid to produce magnesium chloride and hydrogen gas. the reactants in this reaction are?

Magnesium and Hydrochloric acid or Mg and HCl.

The reactants in a reaction are the elements or compounds doing the actual reacting. In this word problem, you can identify this with the keyword reacts. It states, "Magnesium reacts with hydrochloric acid." Thus, you can also infer that the products are what are produced.

Hope this helps!