What is the purpose of a selectable marker in a vector?

The correct answer is Option (3) → To identify and eliminate non-transformants

After transformation, only a few cells take up the vector. A selectable marker is a gene on the vector, usually an antibiotic resistance gene, that lets the transformed cells survive while the others die. It identifies and eliminates non-transformants and lets only the transformants grow.

Option (1) To allow easy linking of foreign DNA — Incorrect. Easy linking of foreign DNA is made possible by the vector's cloning sites, which are very few, preferably single, recognition sites for restriction enzymes. The marker does not do this.

Option (2) To control the copy number of linked DNA — Incorrect. The copy number of the linked DNA is controlled by the origin of replication (ori), not by the selectable marker.

Option (3) To identify and eliminate non-transformants — Correct. When cells are spread on a medium containing the antibiotic, cells that took up the vector carrying the resistance gene grow, and untransformed cells die.

NCERT Extract:
Cloning Vectors – “… This sequence is also responsible for controlling the copy number of the linked DNA. … (ii) Selectable marker : In addition to ‘ori’, the vector requires a selectable marker, which helps in identifying and eliminating non-transformants and selectively permitting the growth of the transformants. Transformation is a procedure through which a piece of DNA is introduced in a host bacterium.”