The correct answer is: D. State model representatives.
State-space methods can be extended to systems which are time-varying. This is because state-space methods are based on the state-space representation of a system, which is a mathematical model that describes the system’s state and how it evolves over time. The state-space representation of a system can be used to represent both time-invariant and time-varying systems.
Bode-Nyquist stability methods, transfer functions, and root locus design are all methods that are used to analyze and design control systems. However, these methods are only applicable to time-invariant systems. This is because these methods are based on the transfer function representation of a system, which is a mathematical model that describes the relationship between the system’s input and output signals. The transfer function representation of a system can only be used to represent time-invariant systems.
Therefore, the only method that can be extended to systems which are time-varying is state-space methods.