Which statement is true about dictionaries?

• A. Dictionaries can only hold numeric values
• B. Keys in dictionaries must be unique
• C. Dictionaries can be defined with square brackets
• D. Dictionaries automatically sort their keys

Answer: (B)

Keys in dictionaries must be unique. This characteristic is fundamental to how dictionaries are structured in Python. A dictionary is a collection of key-value pairs, where each key is used to access its corresponding value. If you try to use a duplicate key when adding a new key-value pair to a dictionary, the previous entry with that key will be overwritten by the new one. This ensures that each key in the dictionary can be mapped to one, and only one, value, maintaining the integrity of the data stored.

The other statements related to dictionaries contain inaccuracies, such as suggesting that dictionaries can only hold numeric values or can be defined with square brackets. In reality, dictionaries can hold values of any data type—strings, lists, other dictionaries, etc. They are defined using curly braces, not square brackets, which are used for lists. Additionally, dictionaries do not automatically sort their keys; they maintain the insertion order as of Python 3.7, but this is not the same as being sorted.