Your choice (conc. HCl) is 

• You need to add concentrated* HCl until the solution becomes acidic. This process is called acidification*.
  
   
• In an acidic solution, 4-methoxybenzoic acid* is completely protonated* and no longer very soluble in aqueous solution.

OK, click ADD to add conc. HCl dropwise until the solution becomes acidic.

Watch what happens after each addition!
ADD

Organic Web Chem University of Alberta