In Python, what best describes a tuple?

A tuple in Python is best described as an immutable collection of items. This means that once a tuple is created, it cannot be modified in terms of adding, removing, or changing elements. The immutability of tuples makes them particularly useful for fixed data sets, where the data structure should remain constant throughout the program's execution.

Because of their invariant nature, tuples can be used as keys in dictionaries and are typically used in situations where the integrity of the data must be preserved. This feature distinguishes tuples from other collection types in Python, such as lists, which are mutable and allow for modification after their creation.

In contrast, the other options describe properties not applicable to tuples. For instance, a mutable collection would allow modifications to its contents, which is not the case for tuples. A variable-length collection suggests that the size can change, which also does not apply, as the number of elements in a tuple is fixed upon creation. Lastly, a dictionary-like collection refers to key-value pairs, unlike tuples, which simply hold a sequence of values without any associated keys.