Which of the Following Is a Definition of the Isoelectric $\alpha$

Which of the following is a definition of the isoelectric point of an amino acid?

A) The pH at which an amino acid has no net charge
B) The pH at which the $\alpha$ -NH₂ group is completely protonated
C) The melting point of an amino acid
D) The conductivity of a 1 M solution of an amino acid in water