Acidity is determined by the stability of the conjugate base. Removal of the marked H in structure (B) places the resulting carbanion on a carbon attached to the cyclopentadiene ring. Delocalisation pushes the negative charge into the ring, completing an aromatic six $\pi$-electron system (cyclopentadienide-like).

The conjugate bases from the other structures are not aromatic and have no comparable stabilisation.

$\therefore$ Structure (B) provides the most acidic H. Option (B).