Spermatia are male gametes of red algae.

Red algae: Red algae, also known as Rhodophyta, are a group of marine algae that are mostly multicellular and photosynthetic. They are commonly found in tropical and subtropical waters, but can also be found in freshwater environments. Red algae are characterized by their red or purplish color, which is due to the presence of pigments called phycoerythrins.

Gametes: Gametes are reproductive cells that are involved in sexual reproduction. In most organisms, there are two types of gametes: male gametes (sperm) and female gametes (eggs). Male gametes are usually smaller and more mobile than female gametes, and they are responsible for fertilizing the female gametes to initiate the process of reproduction.

Spermatia: Spermatia are the male gametes of red algae. They are small, non-motile cells that are produced in special structures called spermatangia. Spermatia are usually released into the water, where they can be carried by currents to reach the female gametes. Once they reach the female gametes, spermatia can fuse with them to form zygotes, which can then develop into new individuals.

Importance of spermatia in red algae: Spermatia play a crucial role in the sexual reproduction of red algae. They are the male reproductive cells that allow for the fusion of gametes and the production of zygotes. Without spermatia, red algae would not be able to reproduce sexually and would rely solely on asexual reproduction methods such as fragmentation or spore formation.

Alternative answer choices:
- Diatoms: Diatoms are a group of photosynthetic algae that are characterized by their intricate silica cell walls. They reproduce asexually through cell division and do not produce spermatia.
- Spermatophyta: Spermatophyta, also known as seed plants, are a diverse group of plants that reproduce through seeds. They have male gametes in the form of pollen grains, not spermatia.
- Euglena: Euglena is a single-celled organism that belongs to the group of euglenoids. It reproduces asexually through cell division and does not produce spermatia.

Therefore, the correct answer is option 'A', red algae. Spermatia are the male gametes of red algae and play a crucial role in their sexual reproduction.