The correct answer is: of.
The preposition “of” is used to indicate the person or thing to whom or for whom something is done. In this case, the speaker is saying that it was unreasonable of the person to leave his job and start a business. This means that the speaker believes that the person did not have a good reason for leaving his job, and that he should have stayed in his job instead.
The other options are incorrect because they do not make sense in the context of the sentence. The preposition “with” is used to indicate that someone is doing something together with someone else. The preposition “in” is used to indicate that something is happening or exists within a particular place or time. The preposition “upon” is used to indicate that something happens or is done as a result of something else.
In conclusion, the correct answer is “of” because it is the only preposition that makes sense in the context of the sentence.