Final answer:
The substance that causes the pH of a solution to decrease is best described as an acid, as it increases the concentration of hydrogen ions in the solution. The correct option is 1.
Step-by-step explanation:
When an unknown substance is added to a solution and the pH decreases, the substance is best described as an acid. This is according to the Arrhenius definition of an acid, which states that an acid is any compound that increases the amount of hydrogen ions (H*) in an aqueous solution.
The pH scale, that ranges from 0 to 14, measures the acidity or alkalinity of a solution. A lower pH means a higher concentration of hydrogen ions and, therefore, a more acidic solution. In contrast, a base would increase the pH of a solution by providing hydroxide ions (OH-) or other negatively charged ions that reduce the concentration of hydrogen ions.
Hence, Option 1 is correct.