___________ is a gaseous plant hormone

The correct option/answer is Ethylene.

Plant hormones, or phytohormones, are naturally occurring chemical messengers that regulate various aspects of plant growth, development, and responses to environmental stimuli. Among the major classes of plant hormones, they can exist in different physical states under physiological conditions. Let us analyze the options to determine why Ethylene is the correct answer:
1. IBA (Indole-3-butyric acid): This is a solid, crystalline substance belonging to the auxin family, commonly used to promote root initiation.
2. Abscisic acid (ABA): This is a weak acid that exists as a crystalline solid in its pure form and is involved in seed dormancy and stress responses.
3. NAA (1-Naphthaleneacetic acid): This is a synthetic plant hormone in the auxin family that is a crystalline solid at room temperature.
4. Ethylene: Unlike the other hormones mentioned, ethylene is a simple hydrocarbon gas with the chemical formula $C_2{H}_4$. It is synthesized by plants and easily diffuses through the air and intercellular spaces, regulating processes such as fruit ripening, flower wilting, and leaf abscission.

Therefore, because of its unique physical state as a gas, Ethylene is recognized as the gaseous plant hormone.