Directions: In the following question, a sentence is given with an underlined phrase that may contain a grammatical error. Select the option from the choices below that correctly replaces the underlined phrase. If the original sentence is grammatically correct, select 'No improvement required'.

The small island nation has been secured its sovereignty after decades of persistent diplomatic negotiations.

The correct option to replace the underlined phrase is had secured its sovereignty.

Explanation and Step-by-Step Analysis:
1. Voice Error (Active vs. Passive): In the original phrase "has been secured its sovereignty", the verb construction is in the passive voice. However, the subject ("The small island nation") is the active agent performing the action of achieving its independence and authority. Therefore, the sentence requires active voice rather than passive voice.
2. Tense & Context: The phrase "after decades of persistent diplomatic negotiations" refers to an achievement completed after an extended period of past efforts, making the past perfect tense "had secured" grammatically appropriate.
3. Analysis of Incorrect Options:
• "has secured its sovereign": Incorrect because "sovereign" is an adjective/noun referring to a monarch, whereas the abstract noun "sovereignty" is required.
• "had win its sovereignty" / "has win its sovereignty": Incorrect because perfect tenses require the past participle form of the verb ("won") following "had" or "has", not the base form "win".
• "No improvement required": Incorrect because the original sentence contains a passive voice error.