How can you check if a key exists in a dictionary?

To determine if a key exists in a dictionary in Python, the correct approach is to use the syntax key in dictionary_name. This expression evaluates to True if the specified key is found in the dictionary, and False if it is not. This method is straightforward, efficient, and utilizes the built-in functionality of Python's dictionary, which allows for fast membership tests due to the underlying hash table structure.

The other choices presented do not represent valid methods for checking the existence of a key in a dictionary. For instance, the method dictionary_name.has_key(key) was a feature found in Python 2, but it is no longer available in Python 3, making it obsolete. Similarly, dictionary_name.contains(key) is not a valid function in Python’s dictionary implementation. Lastly, the expression key exists in dictionary_name does not follow the correct syntax for key checking in Python, as it incorporates incorrect wording instead of using the appropriate keyword “in.”

Thus, using key in dictionary_name is the most effective and contemporary method for checking if a key exists in a dictionary.