Question

Forces can be added together only if they are?

Answer 1

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.

Answer 2

The answer is a) because otherwise the forces can't be added as the objects will be different.