One of the key factors, which makes plasmid the vector in genetic engineering is that

The correct option is B

It has the ability to carry a foreign gene.

Plasmids are small, double stranded, closed circular symbiotic DNA molecules that occur naturally in bacteria outside the bacterial chromosome. They have the capability to self-replicate in the cytoplasm of the bacterial cell and are used as carriers for transferring a fragment of foreign DNA into a suitable host.