In genetic engineering, a DNA segment (gene) of interest is transferred to the host cell through a vector. Consider the following four agents (A-D) in this regard and select the correct option about which one or more of these can be used as a vector/vectors.
(A) A bacterium
(B) Plasmid
(C) Plasmodium
(D) Bacteriophage

Plasmids are low molecular weight extrachromosomal DNA that carries origin of replication, restriction sites and selectable markers to confer readily selectable phenotypic traits on host cells. Plasmids require very few genes for their own replication and rest of it can be deleted and foreign sequences can be added to the plasmid which in turn makes them a suitable candidate to serve as a vector for gene transfer. The presence of 1/3rd non-essential genome that can be replaced with foreign DNA and essence of the genome to be 40,000 and 53,000 bp long for its packaging makes bacteriophages ideal vector. Thus, the correct answer is option A.