Which one is the smallest cell?
(a) Bacteria
(b) Parenchyma
(c) PPLO
(d) Virus

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Hint: Prokaryotes are early primitive anaerobes which consist of single-cell organisms. Size of prokaryotes are very small, ranging from 0.1 micrometres to 500 micrometres. Most of the prokaryotes are parasitic; only a few are useful for the environment.

Complete Answer:
The smallest known prokaryote is mycoplasma which was discovered by E. Nocard and E.R Roux in 1898 in cattle. Mycoplasma like pleuropneumonia like organisms (PPLO) is present in pleural fluids of lungs and causes disease like bovine pleuropneumonia.
Size of mycoplasma ranges between 0.1 to 0.15 micrometre. Mycoplasma lacks a cell wall and membrane-bound organelles, and due to the absence of a cell wall, mycoplasma can change its shape easily.

Additional information: Bacteria were first discovered by Leeuwenhoek in 1676. Bacteria are found in all environments where organic matter is present which is useful for its survival. Size of bacteria ranges from 0.15 to 500 micrometre, the smallest known bacteria is Dialister pneumosintes and largest is Thiomargarita namibiensis. Due to the presence of endospore in bacteria, it transmits easily with the help of air current.
Viruses are disease-causing venomous fluid, which was first discovered by D.J Ivanowsky in tobacco. Size of the virus ranges from 10nm to 1300nm. Viruses are made up of nucleoprotein which is inert toward outside. Viruses are only able to reproduce with the help of the living cell, it survives on the nucleus of the host and performs several functions. Viroids, virusoids and prions are subviral pathogens smaller than the virus. Viroids disease-causing free RNA without nucleoprotein, virusoids are small RNAs inside the protein coat, and prions are made up of proteins only.
So, the correct answer is ‘(c) PPLO’.

Note: The virus is smaller in size than mycoplasma but the virus is not considered as a cell because it does not have its cellular component. In presence of host-virus is living organisms and in absence of a host, viruses are a non-living organism.