Answer:
B
Step-by-step explanation:
The correct answer for the organism with strains that naturally carry a circular, 6.3-kb, 2-micron plasmid would be Saccharomyces cerevisiae.
Most strains of Saccharomyces cerevisiae have been found to possess extrachromosomal elements in their nucleus. This extrachromosomal element is circular in nature and of about 2 microns and 6.3 kb without any known specific advantage or function.
Due to the simple nature of the yeast's plasmid, it has been a major source of study for the understanding and control of how replication works in eukaryotic organisms.
The correct option is B.