Which among the following are the smallest living cells, known without a definite cell wall, pathogenic to plants as well as animals and can survive without oxygen?

Mycoplasmas are the smallest known living organisms ($0.1$ to $0.3\mu \text{m}$). They lack a cell wall, which makes them pleomorphic and naturally resistant to antibiotics like penicillin that target cell wall synthesis. They can survive in anaerobic conditions and are pathogenic.