All of the organisms are important as producers in the food chain and as producers of oxygen, with the exception of a) ciliates. b) cyanobacteria. c) diatoms. d) euglenoids.

The correct answer is option A, that is, ciliates.

The ciliates refer to a group of protozoans featured by the existence of hair-like organelles known as cilia that are similar in composition to eukaryotic flagella, but are in usual shorter and existing in much larger quantities, with a distinct rolling pattern than flagella.

These species are not considered as producers in the food chain and as producers of oxygen due to the absence of chloroplasts.