PPLO( pleuro pneumonia like organisms ) comes under the kingdom monerans . They lack cell wall and have size of 0.1 micrometre. That's why they are smallest.

The correct answer is option 'D' - PPLO.

Explanation:

Introduction to Cells:
Cells are the basic structural and functional units of living organisms. They vary in size, shape, and function. Some cells are visible to the naked eye, while others can only be seen under a microscope. The size of a cell is determined by its structure and the specific functions it performs.

Smallest Cells:
Among the given options, the smallest cell is PPLO (Pleuropneumonia-like organism). PPLO is a type of bacteria that lacks a rigid cell wall and is characterized by its small size and unique structure. It is also known as Mycoplasma.

Comparison of Cell Sizes:
To understand why PPLO is the smallest cell among the given options, let's compare the sizes of the other options:

- Human Nerve Cells: Human nerve cells, also known as neurons, are one of the largest cells in the human body. They can extend up to several feet in length, making them much larger than PPLO.

- Chlamydomonas: Chlamydomonas is a single-celled green alga. While it is smaller than human nerve cells, it is still larger than PPLO.

- Virus: Viruses are not considered cells as they lack cellular structures and cannot reproduce on their own. They are much smaller than cells and can only replicate inside host cells. However, PPLO is still smaller than viruses.

- PPLO: As mentioned earlier, PPLO is a type of bacteria that lacks a cell wall. This lack of a rigid structure allows PPLO to be smaller than other bacterial cells. Its small size enables it to invade host cells and cause diseases.

Conclusion:
In conclusion, among the given options, PPLO is the smallest cell. It is a type of bacteria that lacks a rigid cell wall, allowing it to be smaller than other bacterial cells. Its small size enables it to invade host cells and cause diseases.