Avehicle of known mass accelerates along a straight path. according to newton's second law of motion, what caused this acceleration? an unbalanced force has acted on the vehicle. a balanced force has acted on the vehicle. the mass of the vehicle has changed. the direction of the vehicle has changed.

The answer is, "an unbalance force has acted on the vehicle". The vehicle will not change its states, unless otherwise there is a force acted upon on the  object, that compel  it to move in its original position, or to cause to change its direction of motion. This unbalance force cause the vehicle to move in the direction of the applied force.

Kinetic energy = 1/2mv^2. since the mass is constant we can form a ratio of it's kinetic energy at 60 divided by that at 30 to see ke(60)/ke(30) = +(60^2)/30^2). this may be said to 300% increase.