Which action is important when driving a heavy vehicle on a hill?

When driving a heavy vehicle on a hill, using low gear is crucial for maintaining control. Heavy vehicles have a significant amount of mass, which can make them harder to control, especially on inclines or declines. By shifting to a lower gear, the driver can take advantage of the engine's braking power. This helps in slowing down the vehicle when going downhill and provides better traction and stability when ascending.

Low gear allows the vehicle to use engine resistance to assist with braking, reducing reliance on the brake system alone. This is especially important for preventing brake overheating, which can lead to brake failure. In addition, maintaining control on a hill is necessary to ensure the safety of the driver, the load, and other road users.

The other options do not prioritize safety or effective vehicle control. Increasing speed before the incline could result in loss of control and poor acceleration when the vehicle is under load. Coasting down the hill could lead to excessive speed and an inability to stop safely. Using high gear would make it difficult to manage speed and control effectively, especially with the added weight of the vehicle.