The correct answer is option 4: commercial grain farming. Monoculture refers to the practice of growing only one crop over a large area of land. In commercial grain farming, farmers focus on growing a single type of grain, such as wheat or corn, for profit. This is done on a large scale and often involves the use of machinery and technology to maximize yields. Monoculture in commercial grain farming allows for efficient management, as farmers can specialize in the specific needs of the chosen crop. This approach, however, can also lead to problems such as soil degradation, increased vulnerability to pests and diseases, and reduced biodiversity. It is important to note that while options 1, 2, and 3 may involve some level of crop cultivation, monoculture is not a typical characteristic of these agricultural practices.