The correct option is B Mycoplasma
Ostrich egg is the largest cell in terms of volume and weight.
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.
E. coli cell is about 2 μm in length.