Host bacterium is made competent to accept DNA by treating it with:

Divalent cation

The correct answer is Option (3) → Divalent cation 

Since DNA is a hydrophilic molecule, it cannot pass through cell membranes. In order to force bacteria to take up the plasmid, the bacterial cells must first be made ‘competent’ to take up DNA. This is done by treating them with a specific concentration of a divalent cation, such as calcium, which increases the efficiency with which DNA enters the bacterium through pores in its cell wall.

Recombinant DNA can then be forced into such cells by incubating the cells with recombinant DNA on ice, followed by placing them briefly at 42⁰C (heat shock), and then putting them back on ice. This enables the bacteria to take up the recombinant DNA.

The transformed bacterial cells are plated on an agar medium containing an antibiotic. Only the cells that have successfully taken up and expressed the antibiotic resistance gene (often included in the rDNA) will survive and grow, allowing for the selection of transformed colonies.