From Newton's first law of motion we know that every body continues to be at sate of rest or of uniform motion along a straight line unless and until it is compelled by some external unbalanced force.
It is the unbalanced which creates acceleration in a body.
Newton's second law gives the quantitative definition of force which tells that the net external unbalanced force is the product of mass with acceleration.
Mathematically force = mass×acceleration
⇒ F = ma
Force is a vector quantity.There might be many forces acting on a body at different directions,but they can not produce any acceleration.Several forces may cancel each other. Only the resultant force is responsible for the acceleration of the body.
Hence the correct answer to this question is D.unbalanced force
