What is the primary type of response that involves turning or growing towards a specific stimulus?

The primary type of response that involves turning or growing towards a specific stimulus is tropism. Tropisms are directional growth responses in plants, where they move in relation to environmental stimuli such as light (phototropism), gravity (gravitropism), or water (hydrotropism). This growth occurs due to differential growth rates on different sides of the plant, enabling it to adapt to its environment by maximizing light absorption, improving stability, or enhancing water uptake.

Nastic movements do not involve growth towards a stimulus; instead, they are non-directional movements that occur in response to stimuli such as light or touch. Kinesis refers to a non-directional movement or activity level change in response to a stimulus without a specific direction being favored. Directional responses are a broader category but do not specifically refer to growth as seen in tropisms. Thus, tropism is the most fitting answer regarding directional growth towards a stimulus.