Consider a unity feedback configuration with a plant a $\text{PID}$ controller as shown in the figure. $G\left ( s \right )=\dfrac{1}{\left ( s+1 \right )\left ( s+3 \right )}$ and $C(s)=K\dfrac{\left ( s+3-j \right )\left ( s+3+j \right )}{s}$with $\text{K}$ being scalar. The closed loop is
- only stable for $K>0$
- only stable for $\text{K}$ between $-1$ and $+1$
- only stable for $K<0$
- stable for all values of $\text{K}$
