• Denitrification in Eukaryotes: While denitrification was historically thought to be restricted to bacteria and archaea, recent discoveries established that certain benthic eukaryotic protists can perform anaerobic nitrate respiration:
• The Foraminifer Globobulimina pseudospinescens: This deep-sea benthic foraminifer accumulates high intracellular nitrate concentrations ($\gt 150\text{ mM}$) and performs complete denitrification ($ ext{NO}_3^- \rightarrow ext{N}_2$) in anoxic marine sediment zones, using nitrate as an alternative terminal electron acceptor.
• Other Organisms:
• Dictyostelium discoideum: Cellular slime mold (soil aerobe).
• Batrachochytrium dendrobatidis: Chytrid fungal pathogen of amphibians.
• Ornithocercus magnificus: Marine dinoflagellate.
• Conclusion: Globobulimina pseudospinescens is the eukaryotic denitrifier in anoxic marine sediments (Option 4).

Correct option: (4) Globobulimina pseudospinescens