Choose the part of the sentence that has an error:

The correct answer is Option (1) → No sooner she had realized

No sooner ... than with inversion:

• No sooner ... than shows that one action happened immediately after another.
• When a sentence begins with a negative expression such as No sooner, Hardly or Never, the helping verb comes before the subject (inversion):
  • wrong: No sooner she had realized;
  • right: No sooner had she realized.
• Corrected: No sooner had she realized her blunder than she began to take corrective measures.
  Example: No sooner had the bell rung than the children ran out.

Why the Other Options are Incorrect?

Option 2: her blunder than — Incorrect. "than" is the right partner of "No sooner"; this part has no error.

Option 3: she began — Incorrect. The simple past "began" correctly follows the past perfect "had realized"; no error.

Option 4: to take corrective measures — Incorrect. "began to take" is a correct verb + infinitive pattern; no error.