I'm new to D&D. I was reading about save throws in the Player's Handbook and saw this:
To make a saving throw, roll a d20 and add the
appropriate ability modifier. For example, you use your
Dexterity modifier for a Dexterity saving throw.
What determines what type of saving throw it is? The DM? The attack cast on the PC? Or do you add your highest ability modifier to the d20 rolled?