Final answer:
The farmer is practicing selective breeding, which is a form of genetic modification of crops through human intervention.
Step-by-step explanation:
The farmer who crosses different varieties of corn plants and then selects seeds from the offspring plants with the largest, tastiest corn ears to plant the following season is practicing selective breeding. This is a form of genetic modification of crops through human intervention, in which desirable traits are enhanced and perpetuated. This process of selecting and planting seeds from plants with desired traits has been crucial in the development of crops for thousands of years.