The phosphate functional group in the non cyclic amp molecule of question 21 contains "acidic hydrogens"

Acidic hydrogens shows that the hydrogen atom present in them have ability to release positive ion that is H⁺. So, when the functional group in non cyclic amp (Adenosine monophosphate, it is a nucleotide and also known as 5'-adenylic acid,) molecule contains acidic hydrogens, it means it has the ability to apply the acidic property on the molecule.