Correct Option: C

The given system equation
y′(t) – 4y(t) + y(2t) = x(t) is
● Causal: Since, output depends upon the present and past value of the input signal.
● The coefficient of y(t) having time factor y(2t) is also changing, which makes the system equation non-linear.
● Since, y(2t) is a time compressed circuit, which make the system equation time variant. It may be remember that any compressed or expanded circuit element makes the system equation timevarying.
Hence, alternative (B) is the correct choice.