Answer: when the dipole moment and electric field are parallel
Explanation: The formulae that relates the potential energy of a dipole, dipole moment and strength of electric field is given as
u = p * E cosθ
Where, u= dipole potential energy, p = dipole moment, E = strength of electric field.
The expression is at maximum when θ = 0 (cos 0° = 1)
Hence the function for potential energy will be greatest when θ = 0° which implies that the dipole moment and strength of electric field are parallel to each other.