Which statement will correctly check if a key exists in a dictionary?

• A. if key in dict
• B. if dict.has_key(key)
• C. if dict[key]
• D. if key in dict.keys()

Answer: (A)

The statement that correctly checks if a key exists in a dictionary is the one that uses the in keyword directly with the dictionary. This approach is both straightforward and efficient. When you use key in dict, Python checks for the presence of the specified key in the dictionary. This method leverages Python’s optimized dictionary operations, making it the preferred way to perform this check.

In contrast, other options are either outdated or less efficient. For instance, using dict.has_key(key) is not valid in Python 3, as this method was removed. It's important to be aware that dict[key] will actually raise a KeyError if the key doesn't exist, which doesn't fulfill the requirement of a check; instead, it attempts to access the key directly. Checking with key in dict.keys() works, but it is less efficient because it creates a view of the dictionary keys first. The direct in keyword check is generally the most efficient way to determine if a key exists.