The correct answer is option 4: soil. Hydroponics is a method of growing plants without using traditional soil. Instead, plants are grown in a nutrient-rich water solvent that provides all the necessary nutrients for plant growth. This method allows for more efficient use of resources such as water and fertilizer, as well as better control over growing conditions. With hydroponics, plants can be grown indoors, in greenhouses, or in urban environments with limited space. By removing the soil, plants can still receive the necessary nutrients they need to grow, while also eliminating soil-borne diseases and pests. Thus, hydroponics is a soil-less method of plant cultivation.