diatoms
Hi,
Diatoms are a type of algae, which is actually a very broad term used for thousands of different species. Diatoms are known for their hard silica shell and ornate designs, but they are in the algae "family" if you can even call it that. Eukaryotic means a certain type of more evolutionary advanced cell where there are certain internal structures present. That's in contrast to prokaryotic cells which are primitive like bacteria and stuff.
So diatoms are a eukaryotic algae.