The **cooling system** of the vehicle is crucial to maintain the engine at an appropriate temperature and prevent it from overheating and getting damaged. This system uses a **coolant** that circulates through the engine parts, absorbing heat and keeping them cool. Then, the liquid is cooled in the **radiator** and returns to the engine driven by a **water pump**.

For the cooling system to function properly, it is essential that the coolant reservoir is filled with a mixture of **water and antifreeze**. Antifreeze is important because it prevents the liquid from freezing in cold climates, which could damage the engine or the cooling system. Additionally, antifreeze helps to raise the boiling point of the water, thus improving the system's efficiency in high-temperature conditions.

Therefore, the correct answer is to fill the coolant reservoir with **water and antifreeze**.