Directions: A portion of the following sentence is underlined. Select the option that best replaces the underlined portion. If the sentence is already correct as written, select Option D.

Observing the rare alignment of planets through the telescope was a once-in-the-lifetime event that captivated everyone in attendance.

The correct option is a once-in-a-lifetime event.

Step-by-Step Explanation:

1. Understanding the Idiom:
The phrase "once-in-a-lifetime" is a fixed compound adjective in English meaning extremely rare, unique, or happening only once during a person's life. The standard idiomatic expression specifically uses the indefinite article "a" within the phrase (i.e., "once-in-a-lifetime"), rather than the definite article "the".

2. Analyzing the Original Sentence:
The original underlined text is "a once-in-the-lifetime event". Using "the" inside the hyphenated modifier breaks the standard idiom and is grammatically incorrect.

3. Choice of Indefinite Article:
Although the word "once" starts with the vowel letter 'o', it begins with a consonant sound /w/ ("wun-ss"). In English, we use the article "a" (not "an") before words starting with a consonant sound. Therefore, "a once-in-a-lifetime event" is both phonetically and grammatically sound.

4. Evaluation of Options:
• a once-in-a-lifetime event: Correct. It accurately restores the standard fixed idiom and uses the correct article.
• the once-in-a-lifetime event: Incorrect. The indefinite article "a" at the start is preferred here to introduce a general singular countable event.
• an once-in-a-lifetime event: Incorrect. Uses "an" before a consonant sound (/w/).
• No improvement required: Incorrect. The original sentence contains a grammatical error in the idiomatic expression.