Plant growth regulators

Abscisic acid closes stomata and increases the tolerance of plants to various stresses, so it is called the stress hormone.

During germination gibberellins induce the aleurone layer to make hydrolytic enzymes such as α-amylase, which mobilise the stored food.

Gibberellins are growth promoters: they break seed and bud dormancy, promote seed germination and stem elongation.
Abscisic acid (ABA) is a growth inhibitor: it induces seed and bud dormancy and inhibits seed germination and growth.
Since the effects of ABA are opposite to those of gibberellins, NCERT states that in most situations ABA acts as an antagonist to GAs.
IAA (an auxin) and zeatin (a cytokinin) are growth promoters, and ethylene is not the general antagonist of gibberellins.
Hence the antagonist to gibberellins is ABA.

NCERT, under ethylene, states: 'Ethylene also promotes root growth and root hair formation, thus helping the plants to increase their absorption surface.'
On this basis the answer accepted in the key is ethylene.
Abscisic acid is a growth inhibitor, and gibberellins promote stem elongation and bolting rather than root development.
Note: auxin is also known to help initiate rooting in stem cuttings (NCERT), so 'auxin' is given as the answer in some books; the key of the source PDF (ethylene) is retained here.