Final answer:
A reflex is an unlearned, involuntary action that occurs in response to a stimulus without conscious thought. Reflexes are rapid and involve neural pathways, which are different from the complex patterns of behavior found in instincts, which engage higher brain centers.
Step-by-step explanation:
An unlearned, involuntary action or movement in response to a stimulus that occurs without conscious thought is defined as a reflex (option B). This type of behavior does not require prior learning or experience. For example, when a doctor taps the patellar tendon below the kneecap, it stimulates the nerves leading to the automatic leg extension known as the knee-je rk reflex. Further, the instinctual withdrawal of a hand from a hot surface is also a reflex. Reflex actions are rapid and occur via the neural pathways to and from the spinal cord or brainstem, bypassing the higher brain centers involved in conscious decision-making.
In contrast to reflexes, instincts involve more complex patterns of behavior and are also unlearned. They include activities such as birds migrating or building nests and are triggered by a broader range of stimuli. Instinctual behavior involves whole-body movements and typically engages higher brain centers than reflexes. Learning, on the other hand, involves change and experience, where an organism acquires knowledge and skills through interactions with the environment, distinct from innate reflexes and instincts.