The correct answer is: for.
The preposition “for” is used to indicate the purpose or goal of an action. In this case, the speaker has made all preparations for the marriage, which means that they have done everything that is necessary to make the marriage happen.
The other options are not as accurate. The preposition “to” is used to indicate movement towards a destination. In this case, the speaker has not made any preparations to move towards the marriage. The preposition “of” is used to indicate possession or ownership. In this case, the speaker does not own the marriage. The preposition “on” is used to indicate a location or position. In this case, the speaker has not made any preparations on the marriage.
In conclusion, the correct answer is “for” because it is the only preposition that accurately indicates the purpose of the speaker’s actions.