Which of the following describes a non-directional response to the speed of an organism?

A non-directional response is one where the behavior of an organism does not have a specific direction in relation to a stimulus, but rather is influenced by the intensity of that stimulus. Orthokinesis fits this definition as it describes the speed of movement of an organism in relation to the intensity of a stimulus. In orthokinesis, the rate of movement increases or decreases depending on how strong the stimulus is, but the direction of movement is random.

In contrast, klinokinesis is also a non-directional response, but it relates specifically to changes in the turning rate of an organism rather than speed. Tropism refers to growth responses of plants toward or away from stimuli, typically directional, such as light or gravity. Heliotropism is a specific type of tropism that refers to the directional growth or movement of plants toward sunlight. Thus, orthokinesis is the most accurate choice when describing a non-directional response related to the speed of an organism's movement.