Which one of the following system is nonlinear? where y(t)

Which one of the following system is nonlinear? where y(t) = output; x(t) = input

1. y(t) = 2x(t – 1) – 3x(t – 2) + x(t – 3)
2. y(t) = 5x(t)
3. y(t) = 2x(t – 1) – x(t – 2) – x(t – 4)
4. y(t) = 2x(t) + 3.6

After inspecting all the options given in the question, we conclude that there is constant term in option (D), which makes the system non-linear.