Interestingly enough, there are no primitive types in C#; that's a Java term. I'd probably ask for what definition they're after because I can think of a few:

• Types that have a type alias (that makes them special in the language), but that would include string and object.

• Value types that have a type alias. Closer to what Java does and would include most basic numeric types. But it would exclude System.Half and System.Int128 for example.

• Value types that the runtime knows about and might treat in a special way (e.g. by also having specific IL instructions for them). That would include most basic numeric types (and enums), but probably exclude boolean, decimal and the newer ones.

• Value types in general, as they all function similarly, regardless of whether it's bool, int, ValueTuple<float, ushort> or an enum. But that list is no longer finite.

But perhaps the following discussion about types in C# and .NET was what they're after. Who knows.