What sort of car is the safest option for 18 year old?
Look into car safety ratings
IIHS does the testing and rating in the U.S., but it is worth to take a look at Euro NCAP data too.
The heavier the better
... in case of an accident. However, it should handle well too, so that it doesn't roll-over easily.
Get all the safety options available for the model
Additional airbags for the side impact? Of course...
Buy a car that doesn't run too fast
Young drivers can tend to be daredevils. When you have a car that can't reach 200 kilometers per hour, then you hav ea safe car for your 18 year old.
Get the help of an expert when buying used cars
Some cars will develop safety issues as they get older. You may want a good mechanic to help you pin point any potential issue that could be a source of an accident.
Do not buy an old one
Do not assume that it's not worth to buy a newer car, because they probably are going to bump into something as they are learning. Today's cars have usually many more safety features than those produced just 10 years ago.