Anti-freeze used in a vehicle radiator is a mixture of water and

Anti-freeze is a substance added to the water in a vehicle's radiator to lower its freezing point, preventing the coolant from solidifying in cold weather and potentially cracking the engine block.

Among the options given, propanetriol (glycerol or propane-1,2,3-triol, HOCH\(_2\)CH(OH)CH\(_2\)OH) has historically been used as an anti-freeze. Glycerol is highly soluble in water in all proportions, and when dissolved, it significantly depresses the freezing point of the mixture. This is a colligative property: the dissolved solute particles disrupt the formation of ice crystals, requiring a lower temperature to freeze.

The other options are unsuitable:

• Propanol (CH\(_3\)CH\(_2\)CH\(_2\)OH) is a flammable alcohol not used in radiator systems.
• Propanone (acetone, CH\(_3\)COCH\(_3\)) is a volatile ketone and a solvent, not an anti-freeze.
• Propanal (CH\(_3\)CH\(_2\)CHO) is an aldehyde with no anti-freeze application.

In modern practice, ethylene glycol (ethane-1,2-diol) is more commonly used, but among the four choices provided, propanetriol is the correct answer.