In genetic engineering, the antibiotics are used: To select healthy vectors; As selectable markers; As sequence from where replication starts; To keep the cultures free of infection

Vectors carry an antibiotic-resistant gene which helps to select the recombinant cells from non-recombinant ones as only recombinant cells would exhibit the antibiotic resistance and would be able to survive on antibiotic rich medium. Such genes that the host cell requires for growth under certain conditions and differentiate the recombinant cells from non-recombinant ones are called as selectable markers. Thus, the correct answer is option D.