The scenario described, where a researcher changes the DNA in a soybean plant to make it resistant to a weed killer, is best described as genetic modification (option 1).
Genetic modification, also known as genetic engineering, involves altering the DNA of an organism to introduce specific traits or characteristics. In this case, the researcher would have introduced genes into the soybean plant that confer resistance to the weed killer.
Natural selection (option 2) refers to the process by which organisms with advantageous traits are more likely to survive and reproduce, leading to the evolution of a population over time. While natural selection can play a role in the development of traits like resistance to weed killer, the scenario described specifically involves the intentional manipulation of the plant's DNA by a researcher.
Photosynthesis (option 3) is the process by which plants convert sunlight, water, and carbon dioxide into energy-rich molecules like glucose, using chlorophyll and other pigments. While photosynthesis is a vital process for plant growth and survival, it is not directly related to the modification of DNA or the development of resistance to weed killer.
Plant propagation (option 4) refers to the process of reproducing plants through methods such as seed germination, cutting, grafting, or tissue culture. While plant propagation is a technique used to multiply plants with desirable traits, it does not involve modifying the DNA of the plants themselves.
Therefore, the most appropriate choice to describe the scenario is genetic modification (option 1).