Magnifying object would be the answer to your question
They find them during the ceremony of 12 where they are assigned jobs and spouses
B.) Garden
this is because "on" is one of many words that are commonly in prepositions. Then to find the object of the preposition, you have to ask yourself, "on {or insert whatever your word is here} what", because the object of your preposition is the subject of the preposition. :)
I hope that makes sense!
