• Membrane Filter Sterilization (Cold Sterilization): In microbiological and phytopathological laboratory protocols, heat-labile liquids (such as antibiotic solutions, vitamins, growth regulators, and viral inocula) cannot be autoclaved.
• Standard Pore Size: They are sterilized by membrane filtration using cellulose acetate, nylon, or PES membrane filters with a standard nominal pore diameter of $0.22\,\mu\text{m}$ (commonly specified as $0.2\,\mu\text{m}$).
• Sterility Mechanism: A $0.2\,\mu\text{m}$ pore size physically excludes all vegetative bacterial cells (Brevundimonas diminuta is used as the standard validation challenge), fungi, and mycoplasmas while permitting aqueous solutions to pass through.
• Conclusion: Filters of $0.2\,\mu\text{m}$ pore size are normally used (Option 3).

Correct option: (3) 0.2 um