Final answer:
When a vehicle moves straight forward in spite of steering input, it is due to the phenomenon of inertia. The road surface being slippery does not directly affect the vehicle's ability to move straight.
Step-by-step explanation:
When a vehicle moves straight forward in spite of steering input, it is due to the phenomenon of inertia. Newton's first law of motion states that an object at rest or in motion will continue to stay in that state unless acted upon by an external force. In this case, the vehicle's forward motion is not being affected by any external forces such as faulty steering, misaligned wheels, or inadequate steering force from the driver. The road surface being slippery does not directly affect the vehicle's ability to move straight, as long as there is sufficient traction. Therefore, option 4) The road surface is slippery is the correct answer.