The correct answer is D) assimilation of external DNA into a cell
Explanation:
A bacteria is called transformed when it consist a genetic material that is not it's own but taken from outside the cell. So transformation of bacterial cell defines the uptake of foreign DNA by bacterial cell from its surrounding.
This transformation in bacteria is necessary because it provides bacteria some new and extra genes that help it to survive in the adverse and changing environment.
Transformation can result in getting the antibiotic-resistant gene to the bacteria which protect the bacteria from antibiotics during infection when antibiotic is used against it.
So the right answer is D) assimilation of external DNA into a cell.