Answer by Chakravarthy Kalyan for Which is the most acidic hydrogen in vitamin C

Compare the $\ce{OH}$ bonds in Vitamin C (ascorbic acid) and decide which one is the most acidic. The most acidic proton in ascorbic acid is the one whose conjugate base is most resonance stabilized.

1. Removal of either of these $\ce{H}$'s at hydroxyl group A or B does not give a resonance stabilized anion:

$\bf{Scheme \ 1}$

2. The proton at D is less acidic, since its conjugate base is lessresonance stabilized with two resonance structures:

$\bf{Scheme \ 2}$

3. The proton at C is the most acidic proton in ascorbic acid since the conjugate base is most resonance stabilized:

$\bf{Scheme \ 3}$