Which substance has the lowest [h+] concentration but is still considered an acid?

milk
blood
gastric fluid
household lye

milk

Explanation:

Gastric fluid is highly acidic with a pH of 1.5 -3.5 and very high [H⁺]. Milk has a pH of 6.5-6.7, so it has the lowest [H⁺]. Milk is only slightly acidic, it is very close to neutral pH of 7.0. Milk has lactic acid, which is a weak acid. Blood is neutral with pH of around 7.0,  household lye is alkaline with a pH of 13.5. the higher the pH the lower the [H⁺] and vice versa.

pH = - log [H⁺]