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The protists areunicellular, and one of the photosynthetic protists is diatoms. They are surrounded by the cell walls, which are patterned intricately, and are formed of silicon dioxide. They are commonly found inmarine and freshwater. In diatoms, the mode of reproduction is asexual. However, in some of the cases, sporulation and sexual reproduction are also witnessed.

Thediatoms possess the tendency to produce their food, thus, they are theproducers. They do not require to capture food. In the diatoms, raphe is the composition found in the silica shell, however, it does not have any role in the process of capturing food, defense, or photosynthesis.

Thus, the options b, c, and d are incorrect.

A slit, which is presented by the diatoms in their silica shell is known as raphe. From the raphe, the mucopolysaccharides stream is ejected by the diatoms. This stream helps the diatoms inlocomotion that is, thrusting in one direction.

Hence, the correct answer is option a.