What is true regarding the difference between mutable and immutable objects?

• A. Mutable objects can be hashed, while immutable objects cannot
• B. Immutable objects can hold references to mutable objects
• C. Mutable objects retain their value across program executions
• D. Immutable objects can be modified using the `pop` method

Answer: (B)

The statement that immutable objects can hold references to mutable objects is accurate. Immutable objects, such as integers, strings, and tuples in Python, cannot be changed in place after they are created. However, they can contain references to other objects, including mutable types like lists and dictionaries.

For instance, you could have a tuple that contains a list, and while the tuple itself cannot be changed (you can't change its size or contents), the list inside it can be modified. This reinforces the concept that immutable and mutable types serve different purposes and can co-exist; immutables can encapsulate mutables without altering their immutability.

The other choices present misunderstandings: mutable objects can indeed be hashed, provided they are not changed during their use as keys in a dictionary; mutable objects do not retain their value across program executions—values only persist if explicitly saved to external storage; and immutable objects cannot be modified with methods like pop, which is specific to mutable objects.