When hydrogen chloride ionizes in water, which reactant is the proton donor? chlorine which reactant is the proton acceptor? hydrogen?

Let's write the reaction first.

HCl + H₂O ---> H₃O⁺ + Cl⁻

These reaction has two reactants, either the proton donor or the proton acceptor. Water is amphoteric, meaning it can act as an acid or base. Since HCl is an acid, then water in this reaction acts as a base. 

1. The proton donor is HCl because it donates H+ to water which yields a hydronium ion, H₃O⁺.
2. The proton acceptor is water.

B. a graduated substance