In genetic engineering, the term vector is applied for
A. Plasmids
B. Source of DNA
C. Cell which receives DNA
D. Active disease-causing viruses
E. All of the above

Hint: The term biotechnology refers to the use of living organisms or their products to modify human health and the human environment.
Genetic Engineering: Techniques to alter the chemistry of genetic material (DNA and RNA), to introduce these into host organisms and thus change the phenotype of the host organism.

Complete answer:
Traditional hybridisation procedures used in plant and animal breeding, very often lead to inclusion and multiplication of undesirable genes along with desired genes. The techniques that are involved in genetic engineering include some steps such as creation of recombinant DNA, use of gene cloning and gene transfer overcome this limitation and make us able to separate and introduce only one or a set of desirable genes without introducing undesirable genes into the target organism.
Vector is a carrier/vehicle that delivers a foreign piece of DNA into the host organism and they are engineered in such a way that they help in following Easy linking of foreign DNA. Selection of recombinants from non-recombinants.
Now let us gather information about given options:-
A. Plasmids :- These are extra chromosomal, circular, non-essential, double-stranded, autonomous, self-replicating pieces of DNA in bacterial and some yeast cells. They may confer the property of antibiotic resistance (bacteria) and virulence (Ti plasmid in Agrobacterium tumefaciens). These plasmid DNA act as vectors to transfer the piece of DNA attached to it into the host organism.
B. Source of DNA :- the source of DNA may be called as source cell. It gives DNA that is needed to be monitored and engineered . The DNA is extracted from the source cell and extracted in form of fragments .
C. Cell which receives DNA :- the cell which receives DNA is called a host cell and competent cell. The vector helps to carry the gene into a host cell, which is usually a bacterium although other types of living cell can be used
D. Active disease causing viruses :- the disease causing viruses do not serve as vectors rather they get inside the cell and damages the cell . They can not be used as vectors as if plasmodium will be used as vector as they will cause malaria

All of the above :- this option cannot be true as all the options are not correct.
Our required option is plasmids.

Note: The production of the first recombinant DNA arrived from the possibility of linking a gene-encoding antibiotic resistance with a native plasmid (autonomously replicating circular extrachromosomal DNA) of Salmonella typhimurium. Stanley Cohen and Herbert Boyer made this possible in 1972 by isolating the antibiotic resistance gene by cutting Out a piece of DNA from a plasmid which was responsible for conferring antibiotic resistance.