In a RBD, what kind of relationship is represented when systems are in series?

When systems are arranged in series in a Reliability Block Diagram (RBD), their performance is indeed dependent on each other. This means that the overall reliability of the system is a function of the reliability of each individual component. If one component fails, it directly impacts the entire system's ability to function, as there is no alternate pathway for system operation available.

In a series configuration, the failure of any single component results in the failure of the whole system. This clearly illustrates the inherent dependence among the components, highlighting how interconnected the performance of each individual unit is to the overall system performance. While the other choices mention relationships such as independence or enhanced performance, these do not reflect the series arrangement's characteristic where each block’s output becomes the input for the next, stressing the crucial nature of their interdependence.