Which equation represents a reaction for which a base dissociation constant expression, ${K_b}$, can be written?

A.     ${\text{C}}{{\text{H}}_{\text{3}}}{\text{COOH(aq)}} + {\text{N}}{{\text{H}}_{\text{3}}}{\text{(aq)}} \rightleftharpoons {\text{C}}{{\text{H}}_{\text{3}}}{\text{CO}}{{\text{O}}^ - }{\text{(aq)}} + {\text{NH}}_4^ + {\text{(aq)}}$

B.     ${\text{HF(aq)}} \rightleftharpoons {{\text{H}}^ + }{\text{(aq)}} + {{\text{F}}^ - }{\text{(aq)}}$

C.     ${\text{HCN(aq)}} + {\text{O}}{{\text{H}}^ - }{\text{(aq)}} \rightleftharpoons {\text{C}}{{\text{N}}^ - }{\text{(aq)}} + {{\text{H}}_{\text{2}}}{\text{O(l)}}$

D.     ${\text{N}}{{\text{H}}_{\text{3}}}{\text{(aq)}} + {{\text{H}}_{\text{2}}}{\text{O(l)}} \rightleftharpoons {\text{NH}}_4^ + {\text{(aq)}} + {\text{O}}{{\text{H}}^ - }{\text{(aq)}}$

D