Nucleophiles are electron-rich species that supply a lone pair to form a new bond at an electron-poor carbon, while electrophiles are electron-deficient species that accept such a pair. The hydroxide ion carries a negative charge and three lone pairs on oxygen, making it a classic nucleophile that attacks positive or partially positive centres. Boron trifluoride is incorrect because boron has only six electrons and an empty orbital, so it accepts electron pairs and behaves as a Lewis acid, an electrophile. The nitronium ion is positively charged and electron-seeking, so it is an electrophile used in nitration, not a nucleophile. Aluminium chloride is an electron-deficient Lewis acid that accepts pairs, again an electrophile. The presence of an available lone pair and negative charge identifies OH^- as the nucleophile, matching NCERT definitions. Sanity check: a species that gives electrons must be electron-rich, and the negatively charged hydroxide fits this requirement perfectly.