Soilless agriculture refers to the practice of growing plants without using traditional soil as a medium for their growth. Instead, it involves using various methods to provide the necessary nutrients and water directly to the plant roots. Option 1, hydroponics, is the correct answer because it is a specific method of soilless agriculture. In hydroponics, plants are grown in a nutrient-rich water solution, with their roots submerged or sprayed with the solution. This allows the plants to receive all the essential nutrients they need without soil. Option 2, hygroponics, is not a valid term and does not exist in agriculture. Option 3, sericulture, is the practice of cultivating silk from silkworms and is unrelated to soilless agriculture. Option 4, inter-cropping, is a traditional agricultural practice where two or more crops are grown together in the same area for mutual benefits.