Algae

The wording of this 'question' is frustratingly vague. The fact is that according to Wikipedia, "algae" is "an informal term for a large and diverse ... polyphyletic grouping that includes species from multiple distinct clades. "Polyphyletic" means that the organisms in question "have been grouped together based on characteristics that do not imply that they share a common ancestor that is not also the common ancestor of many other taxa". In other words, they don't all belong to any one taxon.

I have heard this question asked in quizzes. It's a fair enough question, IMHO – just not all that scientific!