Final answer:
The acidic protons in malic acid are the hydrogens attached to the carboxyl (COOH) groups as malic acid is a dicarboxylic acid with two such groups capable of donating a proton.
Step-by-step explanation:
The acidic proton in malic acid refers to the hydrogen atom that is most likely to be donated or dissociated in a chemical reaction, where the acid acts as a proton donor. Malic acid is a dicarboxylic acid, meaning it has two carboxylic acid groups each able to donate a proton. In malic acid, the acidic protons are the ones attached to the carboxyl (COOH) groups. Typically these are the hydrogens that will be the first to ionize in an aqueous solution, making them the acidic protons.
Without specific labels (Proton A, Proton B, etc.), one would need to examine the molecular structure to determine which protons are the most acidic. Generally, acidic protons are found on carbons that are either directly attached to electronegative atoms or groups, or in positions that can stabilize the resulting negative charge after deprotonation. For carboxylic acids, like in malic acid, the hydrogen attached to the oxygen of the -COOH group is the acidic proton.