What is a naturally occurring substance in plants that controls growth and physiological processes?

The correct answer, hormone, refers to a group of naturally occurring substances in plants that regulate growth and various physiological processes. Plant hormones, including auxins, gibberellins, cytokinins, abscisic acid, and ethylene, play crucial roles in traits such as cell division, elongation, and response to environmental stresses.

Hormones function as signaling molecules, allowing plants to coordinate their growth and developmental responses effectively and adapt to their surroundings. For instance, auxins regulate the direction of growth in response to light (phototropism), while gibberellins influence seed germination and flowering.

The other options, while important in plant biology, do not specifically pertain to the natural growth regulators that influence physiological processes in the same way hormones do. Enzymes are biological catalysts facilitating biochemical reactions, carbohydrates serve as energy sources and structural components, and proteins play myriad roles in cellular functions. However, none of these directly act as growth regulators like hormones in plants.