Question

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

Answer 1

when the direction of the dipole(from the -ve to +ve charge)is the same as the direction of the applied electric field,it is in stable equilibrium

Answer 2

  As we keep the dipole with its dipole moment along the direction of the electric field then it will be in stable equilibrium. 

If we keep the same dipole inverted i.e. its dipole moment opposite to the external field then the dipole will be in unstable equilibrium.