The correct option is A as selectable markers
Selectable markers are those genes that help in selecting those host cells which contain vectors (i.e., transformants) and eliminating the non-transformants. The genes encoding resistance to antibiotics such as tetracycline, ampicillin, kanamycin etc., are useful selectable markers for E.coli. Plasmid pBR322 has two resistance genes – ampicillin resistance (ampr) and tetracycline resistance (tetr) which are considered useful for selectable markers.