Which answer is correct?

Use `sum(nested, [])`. The `sum` function concatenates lists when started from `[]`.

— clever_hack

Use `[x for sub in nested for x in sub]`. List comprehension — readable and Pythonic.

— comprehension_fan

Use `functools.reduce(lambda a, b: a + b, nested)`. Functional approach.

— fp_lover

Use `itertools.chain.from_iterable(nested)` wrapped in `list()`. It's lazy, memory-efficient, and handles any iterable of iterables.

— stdlib_user