(a) Stable equilibrium: When angle between $\overrightarrow{\mathit{p}}$ and $\overrightarrow{\mathit{E}}$ is $0^0$

Potential energy of dipole $=-\mathit{p}\mathit{E}\cos \mathit{\theta }$
In this case $\mathit{\theta }=0^{\circ }$, so Potential energy $=-\mathit{p}\mathrm{E}$
(b) Unstable equilibrium: When angle between $\overrightarrow{\mathit{p}}$ and $\overrightarrow{\mathit{E}}$ is ${180}^{\circ }$.

Potential energy of dipole $=-\mathit{p}\mathrm{E}\cos \mathit{\theta }$
In this case $\mathit{\theta }={180}^{\circ }$, so Potential energy $=\mathit{p}\mathrm{E}$