Can an Interface extend another Interface?

Yes an Interface can inherit another Interface, for that matter an Interface can extend more than one Interface.

Can an Interface implement another Interface?

Intefaces doesn't provide implementation hence a interface cannot implement another interface.

Can a method inside a Interface be declared as final?

No not possible. Doing so will result in compilation error. public and abstract are the only applicable modifiers for method declaration in an interface.

Class C implements Interface I containing method m1 and m2 declarations. Class C has provided implementation for method m2. Can i create an object of Class C?

No not possible. Class C should provide implementation for all the methods in the Interface I. Since Class C didn't provide implementation for m1 method, it has to be declared as abstract. Abstract classes can't be instantiated.

Can a abstract class be defined without any abstract methods?

Yes it's possible. This is basically to avoid instance creation of the class.

What is use of a abstract variable?

Variables can't be declared as abstract. only classes and methods can be declared as abstract.

Can you create an object of an abstract class?

Not possible. Abstract classes can't be instantiated.