Which is a valid abstract class?

The method `stop()` is declared without a body and without the `abstract` keyword, making it an illegal declaration for a non-abstract method.

In an interface, methods with bodies, like `start()`, must be declared with the `default` or `static` keyword (from Java 8 onwards); otherwise, all methods are implicitly abstract and cannot have a body.

A method cannot be both `final` (cannot be overridden) and implicitly `abstract` (requires implementation) by lacking a body, as these keywords are contradictory.

This abstract class correctly defines two abstract methods, `drive()` and `stop()`, without implementations, and one concrete method, `start()`, with an implementation. All methods adhere to Java's rules for abstract and concrete method declarations within an abstract class.

A `static` method must provide an implementation (a method body); it cannot be declared without a body, as `static` implies it belongs to the class and doesn't rely on instance state.