Which molecules react to form a dative covalent (coordinate) bond?

A.     \({\text{C}}{{\text{H}}_{\text{4}}}\) and \({\text{N}}{{\text{H}}_{\text{3}}}\)

B.     \({{\text{C}}_{\text{2}}}{{\text{H}}_{\text{2}}}\) and \({\text{C}}{{\text{l}}_{\text{2}}}\)

C.     \({\text{N}}{{\text{H}}_{\text{3}}}\) and HF

D.     Cl2 and HF

C

Students found this question on dative (coordinate) covalent bond to be difficult with 35.13% correct answers. However, choices A and B were commonly selected. In A, the carbon octet is full and cannot bond with the lone electron pair on \({\text{N}}{{\text{H}}_{\text{3}}}\); in B, the reaction of \({\text{C}}{{\text{l}}_{\text{2}}}\) with \({{\text{C}}_{\text{2}}}{{\text{H}}_{\text{2}}}\) would be an addition reaction with covalent bond between C and Cl.