Which of the following solutions has a pH less than 7?

A solution with a pH less than 7 is classified as acidic. Among the options listed, vinegar is a common example of an acidic solution. Vinegar typically has a pH that ranges from about 2 to 3 due to the presence of acetic acid, which contributes to its acidic properties.

Pure water has a neutral pH of approximately 7, making it neither acidic nor basic. Sea water, while slightly alkaline due to dissolved salts and minerals, also maintains a pH above 7. Ammonia is a basic solution, with a pH typically greater than 7, resulting from its ability to accept protons in a solution. Therefore, vinegar is the only option that qualifies for having a pH less than 7, confirming its distinction as an acidic solution.