This is a plant hormone

The correct option is dormin.

Plant hormones, also known as phytohormones, are naturally occurring organic substances that regulate physiological processes in plants. Among the choices provided, dormin is a plant hormone.

Here is a step-by-step breakdown of why this option is correct:
1. Discovery and Identification: In the 1960s, researchers studying the chemical regulation of bud dormancy in plants isolated a growth-inhibiting substance and named it "dormin". Later, it was discovered that dormin is chemically identical to "abscisin II" (a compound promoting leaf abscission). Today, this major plant hormone is universally known as abscisic acid (ABA).
2. Role of Abscisic Acid (Dormin): It acts primarily as an inhibitor of plant growth. It plays vital roles in seed and bud dormancy, regulating stomatal closure during water stress, and helping plants tolerate various environmental stresses.
3. Analysis of Other Options:
- Acetylene: This is a simple hydrocarbon gas ($C_2{H}_2$) commonly used in welding. While it has similar physiological effects to the gaseous plant hormone ethylene, acetylene itself is not a naturally occurring plant hormone.
- Formalin: This is a solution of formaldehyde gas in water, widely used as a disinfectant and biological preservative. It has no hormonal function in plants.

Therefore, dormin is the only plant hormone listed among the choices.