Final answer:
The common method for the genetic improvement of Agaricus bisporus is genetic engineering to introduce desired traits.
Step-by-step explanation:
The common method for the genetic improvement of Agaricus bisporus is option C) Genetic engineering to introduce desired traits. Genetic engineering involves using recombinant DNA technology to modify an organism's DNA and achieve desirable traits. This is done by introducing foreign DNA into the organism through molecular cloning, resulting in a genetically modified organism (GMO).