D? Seems the most reasonable and the rest seem to be realistic.
Often a plasmid is used in recombinant cloning technology to clone newly isolated genes. It is also very common to use a recombinant plasmid to express large amounts of a known gene to obtain RNA or protein from it. Such recombinant gene expression has been indispensable for the biotechnology industry.
A control is a experimental variable that is kept constant or unchanged during an experiment. During a controlled experiment, the controlled variables are held constant to minimize or stabilize their effects on the subject, its unchanging state allows the relationship between the other variables being tested to be better understood.
