Which of the following cannot be defined in an interface?

In Kotlin, an interface can provide declarations for abstract methods, properties, and even default implementations for methods. However, one key point is that properties defined in an interface cannot be mutable, meaning they cannot have a backing field or setter method.

While you can declare properties in an interface, they must either be read-only or have getters that are implemented in the classes that implement the interface. This means that a property that allows for setting or modifying, such as a variable with both a getter and a setter, cannot be defined within an interface's context.

Thus, the option highlighting properties that can be set or modified accurately represents what cannot be done in a Kotlin interface. In contrast, abstract methods without a body, default implementations, and static methods (though static methods per se aren't directly allowed in interfaces, companion objects can provide similar functionality) are all permissible within the constructs and design of Kotlin interfaces.