The correct answer is: A. Signal flow graph.
A signal flow graph is a graphical representation of a system of linear equations. It can be used to represent the transfer function of a system, which is the ratio of the output signal to the input signal.
An analogous table is a table that shows the correspondence between the elements of two systems. It can be used to represent the transfer function of a system by comparing it to a known system.
The output-input ratio is the ratio of the output signal to the input signal. It is not a graphical representation of a system, and therefore cannot be used to obtain the transfer function of a system.
A standard block system is a set of blocks that are used to represent the components of a system. It can be used to represent the transfer function of a system by combining the blocks that represent the components of the system.
Therefore, the only option that can be used to obtain the transfer function of a system is A. Signal flow graph.