The characteristic equation of a linear time-invariant (LTI) system is given by

$$\Delta(s)=s^{4}+3s^{3}+3s^{2}+s+k=0$$

The system is BIBO stable if

1. $0<K<\frac{12}{9}$
2. $k>3$
3. $0<k<\frac{8}{9}$
4. $k>6$
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