Antibiotics are used in genetic engineering. They are useful

The correct option is as selectable markers.

In genetic engineering, selectable markers are essential components of vector DNA (such as plasmids) used to identify and select host cells that have successfully taken up the recombinant DNA (transformants) and eliminate non-transformants.
Antibiotic resistance genes (like those for ampicillin, kanamycin, tetracycline, or chloramphenicol) are commonly engineered into vectors to serve as these selectable markers.

Here is a step-by-step breakdown of how antibiotics act as selectable markers:
1. Vector Construction: A plasmid vector is designed containing an origin of replication, a cloning site, and one or more antibiotic resistance genes (e.g., ampicillin resistance, ${\mathrm{Amp}}_R$).
2. Transformation: The recombinant plasmid is introduced into host bacterial cells (like E. coli). However, this process is not 100% efficient; some cells will take up the plasmid (transformants), while others will not (non-transformants).
3. Selection Process: The mixture of cells is plated on an agar medium containing the corresponding antibiotic (e.g., ampicillin).
4. Outcome: Only the cells containing the plasmid with the resistance gene can survive and grow into colonies on the antibiotic-containing medium. Cells that did not take up the plasmid lack the resistance gene and are killed by the antibiotic. Thus, the antibiotic acts as a selective agent, allowing researchers to easily identify and select transformants.