What is the result of using an undefined key in a dictionary?

• A. A default value is returned
• B. An error occurs
• C. The program terminates
• D. The key is added automatically

Answer: (B)

When using an undefined key in a dictionary, the correct outcome is that an error occurs. In Python, if you attempt to access a value using a key that does not exist in the dictionary, it raises a KeyError. This is a specific type of exception that indicates that the requested key is not found.

For example, if you have a dictionary defined as my_dict = {'a': 1, 'b': 2} and you try to access my_dict['c'], since 'c' is not a defined key in the dictionary, Python will throw a KeyError, indicating that the key you are trying to access does not exist.

This behavior helps in managing access to dictionary elements and encourages programmers to check for key existence or handle such exceptions using try-except blocks, thus maintaining the robustness of the code. In this context, the other options that suggest a default value gets returned, the program terminates, or the key is added automatically do not accurately represent what happens when an undefined key is accessed in a Python dictionary.