Which of the Following Correctly Defines the Isoelectric Point (PI)

Which of the following correctly defines the isoelectric point (pI) of an amino acid or protein?

A) the equilibrium constant for the ionization of the substance
B) the ion constant of water
C) negative log of the concentration of H+
D) pH at which a molecule is electrically neutral
E) pH at which an equivalent distribution of acid and conjugate base exists in solution