The characteristic equation for a system is

$S^{3}+9S^{2}+26S+12(2+K{_c}) = 0$.

Using the Routh test, the value of  $K{_c}$ that will keep the system on the verge of instability is

1. $20.9$
2. $18.4$
3. $17.5$
4. $15.3$