"I drink it."
Translation:Onu ben içerim.
I am just learning myself, but I believe "bu" would be for "this" and "o" for "it". Those are the nominative forms (to be used if "this" or "it" was doing the action of the sentence). The accusative forms are "bunu" and "onu" respectively.
So, using "bunu" would result in "I drink this" instead of "I drink it".
By making O the accustaive onu, it is like a pointer to make sure it is understood that it is O which is being eaten. Now that you have learnt that get ready to use your new found knowledge and get marked wrong again and be left wondering is Duo inconsistent or is Turkish irregular, nobody knows.