Milk
The pH of all given substances is as follow;
Milk = 6.5 to 6.7
Blood = 7.35
Gastric Fluid = 1.5 to 3.5
Household Lye = 13.5
Therefore, we can exclude Blood and Household Lye because there pH is greater than 7 hence, they are either neutral (Blood) or Basic (Household Lye) in nature. Now, we are left with Milk and Gastric Fluids.
As we know greater the concentration of H⁺ ions, smaller is the pH hence, gastric fluid having less pH than Milk will contain more H⁺ ions therefore, more acidic than Milk. While, Milk containing less H⁺ ions (greater pH than Gastric fluid) is less acidic and contains less number of H⁺ ions.
