Forces can be added together only if they are?
a. acting on the same object.
b. balanced forces.
c. unaffected by gravity.
d. substantial.

Force is a vector quantity which has magnitude as well as direction.Before attempting to add the different forces we should very clearly understand the object under consideration first. After the system is chosen,we should count only these forces which are acting only on the object under consideration.

The forces may  be along a line. They may be planar or non-planar ,but they must act only on the same body .If the forces are acting on different bodies ,we can't add them as we can't get a specific direction between them. The objects may be on different plane.The force has no impact on the body under consideration as it has no role in changing the state of  motion or rest of the body.

Hence forces can be added only if they are acting on the .same object.

Hence option 1 is right.

what experimental evidence led scientists to change from the previous model to this one?

a few of the positive particles aimed at a gold foil seemed to bounce back.

the colors of light emitted from heated atoms had very specific energies.

experiments with water vapor showed that elements combine in specific proportions.

cathode rays were bent in the same way whenever a magnet was brought near them.

electromagnetic shielding is the practice of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive or magnetic materials. shielding is typically applied to enclosures to isolate electrical devices from their surroundings, and to cables to isolate wires from the environment through which the cable runs. electromagnetic shielding that blocks radio frequency electromagnetic radiation is also known as rf shielding.

the shielding can reduce the coupling of radio waves, electromagnetic fields and electrostatic fields. a conductive enclosure used to block electrostatic fields is also known as a faraday cage. the amount of reduction depends very much upon the material used, its thickness, the size of the shielded volume and the frequency of the fields of interest and the size, shape and orientation of apertures in a shield to an incident electromagnetic

from wikipedia