The correct answer is: A. decreases.
When a dielectric is placed in an electric field, the field strength decreases. This is because the dielectric material polarizes, which means that the positive charges in the material are attracted to the negative end of the electric field, and the negative charges in the material are attracted to the positive end of the electric field. This creates an electric field inside the material that opposes the external electric field, which reduces the overall field strength.
Option B is incorrect because the field strength decreases, not increases.
Option C is incorrect because the field strength does not reduce to zero.
Option D is incorrect because the field strength does not remain unchanged.