Which of the following is a phylum within Metazoa: Echinodermata, Cnidaria, Rhizopoda, or Platyhelminthes?

• Metazoa is a biological kingdom that includes all multicellular animals. These organisms are typically heterotrophic, meaning they obtain nutrients by consuming other organisms, and most are motile.

• Echinodermata (e.g., starfish, sea urchins) are multicellular animals.
• Cnidaria (e.g., jellyfish, corals) are multicellular animals.
• Rhizopoda is a group of unicellular protists (like amoebas), not multicellular animals. Therefore, they are not metazoans.
• Platyhelminthes (e.g., flatworms) are multicellular animals.

Both Echinodermata, Cnidaria, and Platyhelminthes are phyla that belong to the metazoan group of organisms. Given the options, and assuming a single best answer is expected, Platyhelminthes is a correct example of a metazoan phylum.

The final answer is $\text{Platyhelminthes}$