Isoelectric pH is
The pH value at which the number of positive charges equals the number of negative charges is isoelectric pH of amino acids.

About option 'a'

At its isoelectric pH, an amino acid is in zwitterion form (dipolar molecule), i.e. has both positive and negative charge on the same acid, but the overall molecule is electrically neutral.
But, every amino acid has its specific isoelectric pH at which it will be in zwitterion form. At no pH value, all amino acids will be in zwitterion form.