Here's why:

* Red algae belong to the kingdom Plantae: Just like all other plants, red algae are eukaryotic organisms that obtain energy through photosynthesis.

* They are classified within the phylum Rhodophyta: This phylum encompasses all red algae species, showcasing their unique characteristics like red pigments, a complex life cycle, and their important role in marine ecosystems.

So, while you might hear someone refer to "the kingdom of red algae," it's technically incorrect. Red algae are a specific group within the larger kingdom of plants.