Mycoplasma mycoides is of particular interest because:
it interferes with the ability of viruses to invade mammalian cells
it has multiple nuclei it is one of the smallest of cells with among the smallest of genomes
it was engulfed by ancient cells and became chloroplasts

It has multiple nuclei, It is one of the smallest of cells with among the smallest of genomes.

Explanation:

Mycoplasma mycoides is a bacterial strain of the genus Mycoplasma. It belongs to the class of Mollicutes. This is parasitic in nature. It lives in the ruminants.  It is smallest known bacteria that does not posses the cell wall. It is present everywhere as a pathogen. It's function is to interfere with the ability of the virus to affect the mammalian cells. It posses multiple nuclei.

It is smallest free-living single celled organism. Due to the small size the entire genome can be sequenced. It can be useful for purpose of research and it is of particular interest because of it's small cell size and multiple nuclei. It serves as a model organism to study the bacterial evolution.

what is this saying?

it should be d) transitional fossils.

they show the gradualism of evolution throughout a period of time