Diatoms belong to the following classification: Chrysophytes.

Diatoms, also known as Bacillariophyta, are a type of algae that are classified under the division Chrysophyta. This division includes various types of photosynthetic organisms that have yellow-brown chloroplasts, which give them a golden-brown color. Diatoms are one of the most common types of chrysophytes and are known for their unique glass-like cell walls made of silica.

1. Chrysophytes:
- Chrysophytes are a diverse group of organisms that belong to the division Chrysophyta.
- They are primarily aquatic and can be found in both freshwater and marine environments.
- Chrysophytes can be either unicellular or multicellular.
- They possess chlorophyll a and c, as well as other pigments that give them their characteristic golden-brown color.
- Diatoms are one of the major groups of chrysophytes.

2. Protozoans:
- Protozoans are a diverse group of single-celled eukaryotic organisms.
- They are not photosynthetic and obtain their nutrients by consuming organic matter or other organisms.
- Protozoans are classified under the kingdom Protista and are not related to diatoms.

3. Dinoflagellates:
- Dinoflagellates are another group of unicellular organisms that belong to the kingdom Protista.
- They are characterized by the presence of two flagella, which they use for movement.
- Dinoflagellates can be either photosynthetic or heterotrophic.
- While they share some similarities with diatoms, they are distinct organisms and not classified as diatoms.

4. Euglenoids:
- Euglenoids are a group of unicellular organisms that belong to the phylum Euglenophyta.
- They are characterized by the presence of a flagellum, which they use for movement.
- Euglenoids can be either autotrophic or heterotrophic.
- They are not closely related to diatoms and have different cellular structures.

In conclusion, diatoms belong to the classification of chrysophytes, which is a division of photosynthetic organisms that have golden-brown chloroplasts. They are distinct from protozoans, dinoflagellates, and euglenoids, which belong to different classifications.