When an object moves, stops moving, changes speed, or changes direction, how do scientists describe that condition?

A.
stable factors

B.
frictional force

C.
a balanced set of forces

D.
unbalanced forces

Question

When an object moves, stops moving, changes speed, or changes direction, how do scientists describe that condition?	
 
 A.
stable factors
 
 B.
frictional force
 
 C.
a balanced set of forces
 
 D.
unbalanced forces

nevermind_justschool · Answer

I think it's frictional force, I could be wrong though.

rushwr · Answer

Even I am thinking the same. But first I was with B and D But D is wrong because if it stops moving then the forces are possibly balanced. That's why. !!!! This one  i have doubts.

nevermind_justschool · Answer

I could not explain why I think that the answer is frictional force, it just sounds right to me.