Correct answer is (b) System is time-invariant, causal and stable
Best explanation: Let x1(t) = v (t), then y1 (t) = u {v (t)}
Let x2(t) = k v (t), then y2 (t) = u {k v (t)} = k y1 (t)
Hence, non-linear
y1 (t) = u {v (t)}
y2 (t) = u {v (t-t0)} = y1 (t-t0)
Hence, time-invariant
Since the response at any time depends only on the excitation at time t=t0, and not on any further values, hence causal.