The correct option is A Selectable marker
A cloning vector is a DNA molecule which can replicate autonomously.
In biotechnology, vectors serve as vehicles to carry the gene of interest.
Plasmids are the most commonly used vectors in biotechnology.
Every cloning vector ideally has a selectable marker. The genes encoding resistance to antibiotics are usually considered as selectable markers. They help in identifying and eliminating non-transformants and selectively permitting the growth of the transformants.
Structural genes code for the synthesis of specific polypeptides/proteins.