Final answer:
Lemon juice, with a pH of 4, is considered an acid because it has a pH level lower than 7.
Step-by-step explanation:
The pH scale is used to classify substances as acidic, basic (alkaline), or neutral. A pH level lower than 7 indicates an acidic substance, a pH level higher than 7 indicates a basic substance, and a pH level of exactly 7 indicates a neutral substance. In this list of liquids, water has a pH of 7, which is neutral. Milk has a pH of 11 and hand soap has a pH of 12, both of which are considered basic. Lemon juice, with a pH of 4, is acidic and is the correct answer to the question.