Correct Answer - Option 4 : -k̂
Friction is the resistance force that opposes relative motion between two objects.
- It is not a fundamental force, like gravity or electromagnetism.
- The direction of friction is always in such a way that it opposes the relative motion
Hence, When the body is sliding in a plane in the direction of a unit vector K = k̂, the direction of friction force will be such that it will oppose the relative motion i.e in the direction -k̂.