Concept:A pure line is a group of crop plants that are genetically identical and homozygous for all traits.
Explanation:Pure lines are formed by repeated self-pollination over several generations.
This process is called continuous inbreeding.
Each generation of self-fertilization increases homozygosity in the offspring.
Heterozygous gene combinations are gradually eliminated.
As a result, the crop population becomes genetically uniform and true to type.
No new genetic variation is introduced through selection or crossing.
Therefore, pure lines are produced through continuous inbreeding.
Answer:D. inbreeding