The gaseous plant growth regulator mentioned in the text is Ethylene . Ethylene promotes root growth and root hair formation, which helps to increase the absorption surface .

Functions of other regulators mentioned in the options:

• Auxins (e.g., 2, 4-D) are used to kill dicotyledonous weeds .
• Gibberellins ($GA_3$) are used to speed up the malting process in the brewing industry .
• Cytokinins help overcome apical dominance .