The electric field of a charge is defined by the force on: an electron a probe charge a proton. a source charge.

A probe charge

Explanation:

As we know that electric field intensity is the force experienced by the probe charge which is placed in the electric field region

Here we can say it as

so here that probe charge should be very small so that it will not disturb the electric field in the space.

If the probe charge is of large magnitude then the field will get disturbed and the intensity which is to be measured is different from its actual value.

Also the sign of the probe charge is taken to be positive.

so correct answer here will be

A probe charge