Friction between sliding surfaces is caused by inertia, microwelds, the normal force, or gravity

The normal force.

Explanation:

Friction can be defined as the resistance of movement applied on a object, that's why is always in opposite direction of the movement.

However, the full definition is accomplish by analysing its equation:

Where is the coefficient of friction, which change from 0 to 1, depending on the material of the surface. refers to the normal force, which is the reaction of the weight, according with Newton's third law, its equal and opposite to the weight.

As you can see, friction depends directly on the normal force, the greater is this force, the greater will be the "friction force".

Therefore, the right answer is the third option.

Is a  vector quantity  that refers to "the rate at which an object changes its position." imagine a person moving rapidly - one step forward and one step back - always returning to the original starting position. while this might result in a frenzy of activity, it would result in a zero velocity. because the person always returns to the original position, the motion would never result in a change in position. since velocity is defined as the rate at which the position changes, this motion results in zero velocity. if a person in motion wishes to maximize their velocity, then that person must make every effort to maximize the amount that they are displaced from their original position. every step must go into moving that person further from where he or she started. for certain, the person should never change directions and begin to return to the starting position.

My answer is opptionc, just check with anyone else