Answer:
Carboxyl
Explanation:
The pH scale measures the amount of positive hydrogen ions, henceforth H+, that are in a solution concerning the number of hydroxide ions, labeled OH-.
Most functional groups have little to no effect on the acidity of the molecule, ketone has no hydrogens to donate to solution or places to accept hydrogen, hydroxyl don't usually lose its hydrogen, aldehyde has a hydrogen to lose but it is connected to a carbon molecule and this prevents its losing, sulfhydryl more often likes to find other sulfhydryls to bond with as opposed to donating hydrogen to the solution.
The carboxyl functional group is often referred to as an acid group, with the OH on the end of the carboxy, the double-bonded oxygen usually helps to hoard the electrons and the hydrogen that is attached falls of into solution, lowering the pH, increasing acidity.
In the image I annexed we can see that even though amino acids are also acidic, the molecule of interest has no amino group, therefore we can conclude that the citric acid's acidity comes from their carboxyl groups.
