When blue litmus was added to an unknown liquid in a test tube, it turned red. What was the liquid?

An acid

The correct answer is option 1. An acid.

When blue litmus turns red upon addition to a liquid, it indicates that the liquid is an acid.

Blue litmus paper turns red in the presence of an acidic solution, indicating the presence of hydrogen ions (H⁺). Acids donate protons (H⁺ ions) when dissolved in water, which causes the litmus paper to change color. Therefore, option 1, "An acid," is the correct choice.