The correct answer is option 'A' - Isoelectric point.

Explanation:
The pH value of a solution in which a polar amino acid does not migrate under the influence of an electric field is called the isoelectric point (pI). The isoelectric point is a fundamental property of amino acids and proteins.

- The isoelectric point is the pH at which the net charge of an amino acid or protein is zero. At this pH, the number of positive charges on the amino acid or protein equals the number of negative charges.
- The isoelectric point is specific to each amino acid and protein and is determined by the presence of acidic and basic functional groups in the molecule.
- Amino acids have both acidic and basic functional groups. The carboxyl group (-COOH) is acidic and can donate a proton (H+), while the amino group (-NH2) is basic and can accept a proton.
- In a solution with a pH higher than the isoelectric point, the amino acid or protein will carry a net negative charge. This is because the excess of OH- ions in the solution will deprotonate the carboxyl group, resulting in a negatively charged carboxylate ion (-COO-).
- In a solution with a pH lower than the isoelectric point, the amino acid or protein will carry a net positive charge. This is because the excess of H+ ions in the solution will protonate the amino group, resulting in a positively charged ammonium ion (+NH3).
- At the isoelectric point, the positive and negative charges cancel each other out, resulting in a neutral molecule. This means that the amino acid or protein will not migrate under the influence of an electric field since there is no net charge to attract or repel it.

In summary, the pH value at which a polar amino acid does not migrate under the influence of an electric field is called the isoelectric point (pI). At the isoelectric point, the amino acid or protein carries no net charge, as the positive and negative charges cancel each other out.