Lemon juice turned the cabbage juice pink at about pH 2. Lemon juice contains citric acid, an organic acid, see chemical structure in picture. Lemon juice is weak enough to be eaten without harm, but can cause eye and skin irritation. The molecular structure for citric acid was placed next to the pH 2 display glass.
Note the structure has three acid groups (COOH). The reason this is not as strong as muriatic acid is because the COOH groups do not release all their hydrogens as H+, that determines the acid’s strength.