The correct option is B Unstable
It is given that, U=x3−6x2
We also know that F=−dUdx=−3x2+12x
∴dUdx=3x2−12x
For the condition of equilibrium, force F=0⇒F=−3x2+12x=0
⇒F=0⇒3x(4−x)=0
∴x=0 ,x=4
d2Udx2=ddx(3x2−12x)=6x−12
At (x=0)
∴d2Udx2=−12<0
∵F=0 & d2Udx2<0 is the condition for unstable equilibrium
⇒ Unstable Equillibrium