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