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