Final answer:
A substance is considered acidic if it has a pH of less than 7.
Step-by-step explanation:
A substance is considered acidic if it has a pH of less than 7. The pH scale ranges from 0 to 14, with 7 being neutral. If the pH is lower than 7, it means that the substance has a higher concentration of hydronium ions than pure water and is therefore acidic.
Based on the given options, the correct answer would be C) 7. A pH value of less than 7 indicates acidity.