Final answer:
The correct answer to the question regarding behavior being entirely determined by external environment is 'determinism'.
Step-by-step explanation:
The idea that behavior is entirely determined by the external environment is called determinism. In the context of behaviorism and personality theory, this concept, particularly as it relates to the work of B.F. Skinner, suggests that our actions and behavior patterns are shaped by the reinforcements and consequences we experience in our environment. This stands in contrast to reciprocal determinism proposed by Albert Bandura, where behavior, cognitive processes, and the situational context influence each other. Hence, the best answer from the choices provided is (a) determinism.