Red tide is caused by

The correct answer is All of the above.

Red tide is a common name for a phenomenon known as an algal bloom, which is an event where marine or freshwater algae accumulate rapidly in the water column, resulting in the coloration of the surface water. This phenomenon is most frequently caused by specific microscopic, single-celled eukaryotic organisms known as dinoflagellates.

Let us look at each of the organisms listed in the options:
1. Gonyaulax: This is a prominent genus of dinoflagellates. It is one of the most well-known organisms responsible for producing marine red tides. When they multiply rapidly, they make the sea appear red and release toxins that can accumulate in shellfish, causing paralytic shellfish poisoning (PSP) in humans and marine life.
2. Noctiluca: Often called the "sea sparkle," Noctiluca scintillans is a free-living, marine-dwelling species of dinoflagellate that exhibits bioluminescence. Blooms of Noctiluca can cause thick green or red discolorations of the water, representing another classic cause of red tides.
3. Ceratium: This is another genus of dinoflagellates characterized by their armor-like plates and horn-like projections. Under favorable environmental conditions, species of Ceratium can undergo rapid multiplication (blooms), leading to red or brown tides that deplete oxygen levels in the water and harm aquatic life.

Since all three organisms—Noctiluca, Ceratium, and Gonyaulax—are dinoflagellates capable of causing red tides, the correct option is "All of the above".