Answer: electrostatic and gravitational force
Mechanical energy remains constant (conserved) if only conservative forces act on the particles.
In this sense, the following forces are conservative:
-Gravitational
-Elastic
-Electrostatics
While the Friction Force and the Magnetic Force are not.
According to this, mechanical energy is conserved in the presence of electrostatic and gravitational forces. Therefore the correct options are A and D.