He spoke . . . . . . . . the subject for two hours regularly. A. in B. on C. of D. with

The correct answer is C. of.

The preposition “of” is used to indicate the subject of a conversation or discussion. In this case, the speaker is talking about a specific subject for two hours regularly. Therefore, the preposition “of” is the most appropriate choice.

The other options are not as accurate. The preposition “in” is used to indicate a location or position. The preposition “on” is used to indicate a surface or area. The preposition “with” is used to indicate a person or thing that is associated with or involved in something.

Here are some examples of how the preposition “of” is used to indicate the subject of a conversation or discussion:

• We talked of the weather.
• I’m thinking of buying a new car.
• What do you think of the new movie?

I hope this helps! Let me know if you have any other questions.