Final answer:
The Modern approach to management recognizes the complexity and dynamic nature of human and organizational behavior, differing from the more rigid views of other approaches like Theory X.
Step-by-step explanation:
The major approach to management thinking that assumes people and organizational systems are complex and ever-changing is the Modern approach. Unlike the Classical, Neoclassical, and Administrative approaches, the Modern approach acknowledges and incorporates the complexity of human behavior and dynamic organizational environments into its management philosophy. This contrasts with approaches like Theory X, which views employees as inherently disliking work and being primarily motivated by external rewards such as pay.