Which of the following describes directional growth towards or away from a stimulus?

Directional growth towards or away from a stimulus is best described by tropism. Tropism refers specifically to a plant's growth response that is directed by the direction of a stimulus. For example, phototropism is when plants grow towards light, while gravitropism involves growth in response to gravity. The fundamental characteristic of tropism is that it involves growth in a specific direction as a response to an environmental stimulus.

Other options do represent types of movements or responses to stimuli, but they do not capture the essence of directional growth as specifically as tropism. Nastic movements are non-directional responses that occur regardless of the stimulus direction, while kinesis refers to a movement in response to a stimulus that does not involve orientation, as it might speed up or slow down movement without growing towards or away from the stimulus. Phototropism is indeed a type of tropism but does not encompass the broader definition of directional growth in response to any type of stimulus.