Answer:- Convergent
Explanation :-
- Convergent evolution is the process in which the organisms that are not closely related evolve independently in such a way that they acquire similar traits due to similar environment in which they live or the ecological niche that they occupy.
- Such structures that result from convergent evolution are called as analogous structures, which perform similar function but have different origins.
- Convergent evolution can be distinguished from parallel evolution in the fact that in parallel evolution similar traits develop in different species which shared a common ancestor and hence, are closely related.
Thus, since the brown and green algae look similar but are not close relatives they show convergent evolution.