The correct answer is D. in.
The word “in” is the most common preposition used to indicate location. It is used to describe the position of something inside a container, building, or other enclosed space. In this case, the sentence is about a boy who is not in the class. The word “in” is the most logical choice to indicate that the boy is not inside the classroom.
The other options are not as logical. The word “on” is used to indicate location on a surface. The word “to” is used to indicate movement towards a destination. The word “into” is used to indicate movement into a container or enclosed space. None of these options make sense in the context of the sentence.
Therefore, the correct answer is D. in.