The correct option is
B Slime moulds
Diatoms are the primary photosynthetic producers in marine food chains.
Dianoflagellates are mostly autotrophic but few are parasitic or symbiotic.
Euglenoids are unicellular phototrophic eukaryotes that also exhibit a facultative saprophytic mode of nutrition.
But slime moulds are the eukaryotic organisms that live freely as single or in the group. They feed on dead or decaying organisms and thus called as decomposers.
Therefore, the correct answer is option C.