Which statement describes a nonlinear system?

• A. Output is not directly proportional to input
• B. Output is directly proportional to input
• C. Output is always zero
• D. Output is always proportional to the square of input

Answer: (A)

Nonlinear systems do not have an output that scales with a constant gain as the input changes. In a linear system, the output is proportional to the input, so the relationship is a straight line through the origin and doubling the input doubles the output. The statement that the output is not directly proportional to the input captures the general idea of nonlinearity: the transfer curve can bend, saturate, or change gain, so there isn’t a single multiplier that relates input and output across all levels.

The other descriptions describe either linear behavior (constant proportionality), a degenerate linear case (output always zero, which is a trivial linear system with zero gain), or a specific nonlinear form (output proportional to the square of input). While the latter is also nonlinear, the broader, defining feature of a nonlinear system is the lack of direct proportionality between input and output.