Answer: D. A nucleophile supplies both electrons in the new bond. The figure shows an O—H bond in water and the loss of the H—Cl bond; therefore O in OH⁻ supplies the pair to H in HCl.

A: Reverses the donor role: HCl provides the H atom, but does not supply the pair that forms the new bond.
B: Identifies OH⁻ as the donor but mistakes the Cl atom, which leaves as chloride, for the atom receiving the pair.
C: Treats the negatively charged chloride product as the nucleophile; it is formed after the O—H bond is made.

Remember: identify a nucleophile by the source of the bonding pair, not merely by its charge.