The correct option is B 2:1
In △s AOB and COM
∠AOB=∠COM (vertically opposite angles)
∠OAB=∠OCM (Alternate angles)
Thus, △AOB∼△COM (AA rule)
ABMC=AOOC=OBOM⇒21=AOOC=OBOM (Since, M is the mid point of CD and AB = CD)
Now In △s AON and COB
∠AON=∠COB (Vertically opposite angles)
∠ANO=∠OBC (Alternate angles)
Thus, △AON∼△COB (AA rule)
Hence, AOOC=ONOB⇒ONOB=2⇒ON:OB=2:1.