The correct answer is option 2: Crop rotation.

Crop rotation is an agricultural practice where different crops are grown in a specific sequence on the same piece of land over multiple seasons. This practice is followed to improve soil fertility, control pests and diseases, and manage weeds effectively.

Option 1, multiple cropping, refers to the practice of growing two or more crops simultaneously on the same land. This is not the practice described in the question.

Option 3, fallow field, refers to leaving a field unplanted for a period of time to allow the soil to rest and regain fertility. This is not the practice described in the question.

Option 4, mixed cropping, refers to growing two or more different crops together in the same field. While this is a common practice, it is not specifically mentioned in the question.

In conclusion, the farmer described in the question is following the agricultural practice of crop rotation by sowing beans after harvesting a crop of wheat.