The correct option is C Mycoplasma
Mycoplasma is the smallest known living organism. It is about 200 - 300 nm in width.
Viruses are 20 nm in width. Though viruses are smaller than Mycoplasma, viruses are technically not considered to be alive.
Ostrich egg is the largest cell in terms of volume and weight.
E. coli cell is about 2 μm in length