Why Is "Motivation" Such a Complex Concept for I/O Psychologists

Why is "motivation" such a complex concept for I/O psychologists to study?

A) Although the idea of "motivation" seems critical to organizations, it is difficult to determine what will motivate a single individual because it differs across people and across time for the same individual.
B) Motivation has never been properly defined by psychologists, so it is a brand-new field specific to I/O psychology.
C) It is impossible to get accurate answers about the motivation of employees because they are unlikely to be truthful on surveys regarding work-related issues.
D) It's not that the concept is complex, but because I/O psychologists cannot do any controlled studies, it is nearly impossible for them to isolate the relevant variables.