It was indeed unreasonable …… him to leave this job and start business. A. in B. with C. upon D. of

The correct answer is: of.

The preposition “of” is used to indicate the source or origin of something. In this case, it is used to indicate that the decision to leave his job and start a business was made by him.

The other options are incorrect because they do not make sense in the context of the sentence.

• “In” is used to indicate a location or position. It would not make sense to say that the decision to leave his job and start a business was made “in” him.
• “With” is used to indicate a relationship or association. It would not make sense to say that the decision to leave his job and start a business was made “with” him.
• “Upon” is used to indicate a time or occasion. It would not make sense to say that the decision to leave his job and start a business was made “upon” him.