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.