Fill in the blank with the appropriate preposition:

He's interested ..... Maths.

in

The correct preposition to use in the blank is in. The preposition in is used to indicate interest or involvement in something. In this case, he's interested in Maths, which means that he likes Maths and wants to learn more about it.

The other options are incorrect.

• With is used to indicate association or cooperation.
• On is used to indicate location or position.
• At can be used to indicate something that is being discussed or talked about, or it can be used to indicate movement or placement near something.