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The acidic hydrogen is the hydrogen atom that can be easily release from the compound and will make the solution acidic.

More the stability of conjugate base more easily proton is removed from molecule.

In this case when the proton from methyl group at C-8 is removed then the electrons left are delocalized onto the oxygen at C-2 or C-4 position making the conjugate base more stable and hence the proton more acidic.

Proton removal from C-8 methyl group

Here the electron left after proton removal are able to delocalize only on less electronegative atom that is carbon which have less ability to accommodate electron and hence making it less stable so hydrogen at C-7 methyl group are less acidic.

Proton removal from C-7 methyl group