Cell theory is not applicable to
Bacteria
Fungus
Algae
Virus
Viruses lack the conventional structure of a cell and do not execute any cell tasks, hence cell theory does not apply to viruses. Therefore, the correct answer is option 4 which is Virus.
Option (A),(B), and (C) are incorrect because bacteria, algae, and fungi are unicellular and multicellular creatures that have distinct cellular structures with cell organelles, generate energy by burning food, and reproduce. As a result, all living entities fall under the purview of cell theory.