Why can't you upcast a List<Apple> to a List<Fruit>?

Upcasting is the process of converting an object of a subtype to its supertype. It is usually allowed when working with inheritance, but it is not possible with generic types by default. This is because generics are invariant in Java, meaning that subtypes cannot be used in place of their supertype. So, even though an Apple is a subtype of Fruit, a List cannot be used as a List. This is because it would violate type safety as the List could end up containing other types of Fruit that are not Apples. Therefore, options A, B, and C are incorrect because they do not address the specific reason why upcasting a List to a List is not allowed in Java.