In which technique the plants are grown with their roots suspended in the air ?

Aeroponics 

The correct answer is Option (2) - Aeroponics
Aeroponics is the process of growing plants in an air or mist environment without the use of soil or misting the roots with hydroponic solutions, which are suspended in the air.

It does not use soil or aggregate medium. The microbes on the plant are in an oxygen-rich environment.

This allows the microbes time to digest the nutrients and to make them immediately available for the plant’s use.

Well-circulating CO₂ in the room also enhances growth.