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Acidity refers to a substance's ability to donate/liberate ${\mathbf{H}}^{\mathbf{+}}$ ions.

Electronegativity refers to an atom's ability to pull the electron density from the electrons shared (i.e., the bond) towards itself.

The acidity of a species depends on the electronegativity of the atom to which the hydrogen atom is attached.

The higher the atom's electronegativity, the more strongly it pulls the electron density of the bond towards itself. This makes it easier for the bond to dissociate, leading to the liberation of ${\mathrm{H}}^{+}$ions.

The highlighted hydrogen atoms are bound to nitrogen, oxygen, and sulfur atoms in the given molecule.

Oxygen is the most electronegative among these, followed by nitrogen, and then sulfur (least electronegative).

Since acidity is directly related to the electronegativity of the central atom, the hydrogen atom attached to oxygen will be the most acidic, followed by the one attached to nitrogen, and then the one attached to sulfur.

The following figure depicts the acidity of hydrogen atoms, with 1 being the most acidic and 3 being the least.

Acidity of hydrogen atoms