Q.Which of the following species does not acts as a nucleophile?

Nucleophiles ('nucleus-loving') are electron-rich reagents with a strong affinity for electropositive centres -- they possess an unshared (lone) pair of electrons that they 'seek' to share with an electron-poor atom, forming a new covalent bond and becoming stabilised in the process. They are usually negatively-charged ions or electron-rich neutral molecules, and every Lewis base is a nucleophile. Examples span neutral lone-pair donors (ammonia/amines via N:, water/alcohols/ethers via O:, $H_2S$/thiols via S:) and negatively-charged species (halides $X^-$, hydroxide/alkoxide/carboxylate via $O^-$, cyanide $CN^-$).

Electrophiles ('electron-loving') are the electronic opposite -- electron-deficient reagents attracted to negative charge or electron-rich centres, either positively charged or neutral with an empty/accessible orbital (e.g. $SnCl_4$'s vacant d-orbitals). Every Lewis acid is an electrophile. Examples span neutral electrophiles ($CO_2$, dichlorocarbene $:CCl_2$, and the Lewis-acid trio $AlCl_3$/$BF_3$/$FeCl_3$) and positively-charged electrophiles (carbocations $R^+$, the proton $H^+$, alkyl halides via $X^+$, oxonium $H_3O^+$ and nitrosonium $NO^+$, and the nitronium ion $^+NO_2$, the active species in nitration).

This nucleophile/electrophile classification is the electronic lens through which every substitution and addition reaction in this unit is sub-classified (nucleophilic, electrophilic, or free-radical), since it is precisely the reagent's electron-rich or electron-poor character that determines how it will attack a substrate. …