Question: What is the dependency inversion principle?

1. Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program.
2. A class should have only a single responsibility?that is, only changes to one part of the software's specification should be able to affect the specification of the class.
3. Software entities should be open for extension, but closed for modification.
4. Entities must depend on abstractions, not on concrete implementations.

Answer: The correct answer of the above question is Option D:Entities must depend on abstractions, not on concrete implementations.