In what scenario will using a tuple as a dictionary key fail?

Using a tuple as a dictionary key fails when the tuple contains mutable elements. In Python, dictionary keys must be hashable, and this requirement is fulfilled by immutable objects. A tuple itself is immutable, which means you cannot change its contents after creation. However, if the tuple consists of mutable elements, such as lists or dictionaries, the overall hashability of the tuple is compromised because these mutable elements can change, resulting in a different hash value. If the hash value of a key can change, it leads to inconsistencies in the way dictionaries work, as dictionaries rely on these hash values for fast access to elements.

The other options do not cause the use of a tuple as a dictionary key to fail. A tuple with only one element is still a valid key, provided you properly define it (using a trailing comma). An empty tuple is also hashable and can serve as a valid dictionary key. Similarly, a tuple containing strings is entirely valid, as strings are immutable and hashable. The issue specifically arises in the presence of mutable elements within the tuple.