Chemistry - GOC Question with Solution | TestHub

Removing ${\mathrm{H}}_1$ puts the negative charge on the nitrogen attached to the ring :

Removing ${\mathrm{H}}_2$ puts the negative charge on the benzylic nitrogen :

The anion on the ring nitrogen is delocalised into the aromatic ring by resonance, while the charge on the benzylic nitrogen is localised. Delocalisation makes the first conjugate base more stable, so the aryl N-H proton ${\mathrm{H}}_1$ is more acidic.