Which pH level indicates that a substance is highly acidic?

A pH level that indicates a substance is highly acidic typically ranges from 0 to below 7, with lower values indicating increasing acidity. Therefore, the correct answer, which represents substances with a pH below 7, is classified as acidic. This characteristic means that the higher the concentration of hydrogen ions (H⁺) present in the solution, the lower the pH value, signifying a more acidic environment.

In contrast, a neutral pH is exactly 7, indicating a balance between hydrogen ions and hydroxide ions. Basic and alkaline refer to pH levels above 7, indicating that the substance is less acidic and may even have the potential to accept hydrogen ions. Thus, understanding these pH classifications helps to distinguish the levels of acidity or basicity of various substances accurately.