Diatom are aquatic unicellular protist with a transparent, opaline silica cell wall made of hydrated silicon dioxide. Diatom forms a major group of producer in the aquatic environment hence they must stay in well-lit surface waters necessary for photosynthesis converting light energy to chemical energy. Diatoms are nonmotile on it's own with a characteristic ability to resist physical damage from wave action via the ability to grow in colony
Diatoms are unicellular algae and they goes by the Scientific name of Bacillariophyceae. Diatoms belongs to the class of Bacillariophyceae and phylum of Ochrophyta and it can photosynthesize (it has pigment bearing plastids). They are found in all moist places and in the soil. If we are to classify diatoms into order they can be divided into two main order which are; Bacillariales and Biddulphiales.
So, if you look at the question we are told that the aquatic protist that is a primary producer and it must stay in well-lit surface waters. If we look at the Explanation in the paragraph above we can see that DIATOMS match this description.
