Which organism is a photosynthetic protist often responsible for red tides?

• A. Angiosperm
• B. Dinoflagellate
• C. Chlorophyta
• D. Foraminifera

Answer: (B)

The main idea is identifying a photosynthetic protist that commonly causes red tides. Dinoflagellates are single-celled marine protists that perform photosynthesis and can reach very high densities in warm, nutrient-rich coastal waters. When they bloom, their pigments—along with the sheer number of cells—color the water, often giving a reddish hue. Some dinoflagellate blooms are also toxic, producing compounds that harm marine life and can affect human health, which is why red tides are of particular concern.

Angiosperms are flowering land plants, not marine protists, so they don’t fit the scenario. Green algae (Chlorophyta) are photosynthetic and protists, but they aren’t the organisms most commonly linked to red tides in the ocean. Foraminifera are mostly heterotrophic protists with calcium carbonate shells and aren’t known for forming the red-tide blooms associated with photosynthetic dinoflagellates.

So the dinoflagellate best matches a photosynthetic protist frequently responsible for red tides.