which orientation of an electric dipole in a uniform electric field corresponds to stable equilibrium?

The following orientation will give the most stable equilibrium.

This is the most stable configuration of an electric dipole in a uniform electric field. The length of the dipole makes 0⁰ with the electric field.

The potential energy of the dipole at this position is,

U = -p.E = -pE cos0 = -pE

Which is the minimum potential energy (stable equilibrium) of an electric dipole.