Answer:
OSMOSIS
Explanation:
Osmosis (/ɒzˈmoʊ.sɪs/) is the spontaneous net movement or diffusion of solvent molecules through a selectively permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region of higher solute concentration),
 
        
                    
             
        
        
        
pH decreases as the hydrogen ion concentration increases.
<u>Explanation:</u>
When there is a decrease in pH, that is pH decreases from 6 to 3 then the acidity increases. 
That is the pH is between 1 to 7 then it is acidic
When the pH is 7 then it is neutral
When the pH is between  7 to 14 then it is basic 
As the H⁺ ion concentration increases, then the pH value decreases, here pH decreases from 6 to 3. 
So the concentration of Hydrogen ion increases, pH decreases. 
 
        
             
        
        
        
The correct answer would be the last one.