The question asks which one of the options does not contain a COOH group. COOH stands for carboxylic acid group, which consists of a carboxyl group that includes a carbon atom double bonded to an oxygen atom and single bonded to a hydroxyl (-OH) group. This functional group is responsible for the acidic properties of carboxylic acids.

Option 1: Citric acid contains three COOH groups, making it a triprotic acid.

Option 2: Lactic acid contains one COOH group, making it a monoprotic acid.

Option 3: Tartaric acid also contains two COOH groups, making it a diprotic acid.

Option 4: Ascorbic acid, also known as vitamin C, does not contain a COOH group. Instead, it contains a hydroxyl (-OH) group and a lactone ring structure.

Therefore, the correct answer is option 4, Ascorbic acid, as it does not contain a COOH group.