the solubility of AgCl in HCl is

a. low, as Cl- ions cause an equilibrium shift towards the solid form
b. low, as Ag+ has already reacted with Cl-
c. high, as HCl readily ionizes
d. high, as AgCl can then form
e. high, as H2 forms, then escapes

(a) The scant amount of Ag+ is in solution will just combine with the added Cl- to form more AgCl solid.

a is the correct answer. Think Le Charelier's Principle. The solubility is
AgCl(s) ==> Ag^+(aq) + Cl^-(aq)

When HCl is added, it increase (Cl^-) on the right so the reaction shifts to the left making some AgCl ppt and the Ag^+ is decreased. Therefore, the solubility is decreased.