Directions: In the sentence below, the underlined part may contain an error. Choose the option that correctly replaces it. If no correction is needed, select “No improvement required.”

The ancient sandstone arch was formed over million of years through wind and rain.

The correct answer is formed over millions of years.

Detailed Explanation:
Let us analyze the logical reasoning and grammatical structure of the sentence step-by-step.

1. Identifying the Plural Noun Error:
In English grammar, when referring to an indefinite, large number of items or units of time, words like hundred, thousand, million, and billion must be in their plural forms when followed by the preposition "of" (for example, thousands of miles, millions of years). Therefore, the phrase "million of years" in the original sentence is grammatically incorrect and must be changed to "millions of years".

2. Preserving the Correct Verb Form:
The sentence describes an action completed in the past using the passive voice construction: auxiliary verb "was" + past participle verb "formed". Therefore, the past participle formed must be retained to maintain grammatical agreement with "was" (i.e., was formed).

3. Evaluating the Options:
- forms over millions of years: Incorrect. Using the present tense verb "forms" after "was" creates an ungrammatical combination ("was forms").
- form over millions of years: Incorrect. The base form "form" cannot directly follow the past auxiliary verb "was" ("was form").
- No improvement required: Incorrect. The original underlined portion contains the singular error "million of years".
- formed over millions of years: Correct. It correctly uses the plural "millions of years" while retaining the necessary past participle "formed".
- formed over 1 years: Incorrect. The numerical value "1" requires the singular noun "year" rather than "years", and it incorrectly changes the meaning of the original statement.

Therefore, replacing the underlined phrase with formed over millions of years yields a grammatically correct sentence.