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 ?

The correct option is C Mycoplasma
The Mycoplasma are organisms that completely lack a cell wall. They are the smallest living cells known and can survive without oxygen. Many mycoplasma are pathogenic in animals and plants.

Bacillus is a rod-shaped, aerobic or facultative anaerobes, which belongs to a group of the genus of Gram-positive bacteria. These bacteria are widely found in soil and water.

Pseudomonas is a bacteria which helps in the denitrification step of nitrogen cycle. It is aerobic in nature and capable of synthesizing vitamin B.

Nostoc are prokaryotic bacteria lacking membrane-bound organelles.They are kept in cyanobacteria as they are photosynthetic.

So, the correct answer is "option C"