The correct answer is option 2: Static electricity.

Static electricity is the build-up of electric charge on the surface of an insulator. An insulator is a material that does not easily allow the flow of electric current. Unlike conductors, insulators hold their electrons tightly and do not readily allow the movement of charges through them.

When an insulator comes into contact with another material with different electron affinities, such as rubbing a balloon on hair, electrons can be transferred, leaving one material positively charged and the other negatively charged. This transfer of electrons creates static electricity.

On the other hand, option 1, current electricity, flows through conductors and not through insulators. Option 3, magnetic field, does not directly charge an insulator, but it can influence the movement of charges within the insulator. Similarly, option 4, gravitational field, does not have a direct effect on the charging of an insulator.

In conclusion, only static electricity, which involves the buildup of electric charge on an insulator`s surface, can charge an insulator.