Question 4

The acidity of a hydrogen atom in a hydrocarbon is determined by the stability of the conjugate base (carbanion) formed after the removal of the proton ($H^+$). In option (B), the removal of the depicted hydrogen results in a carbanion where the negative charge is directly adjacent to a five-membered ring system. This negative charge can delocalize into the ring, forming a cyclopentadienyl anion. The cyclopentadienyl anion is aromatic because it is planar, cyclic, fully conjugated, and contains $6\pi$ electrons ($4n + 2$ where $n=1$). The stabilization gained through aromaticity makes the conjugate base extremely stable, and thus the corresponding hydrogen is the most acidic among the given options.