In a rural area along the border between Oklahoma and Texas, amateur astronomers gather each year to observe the night...

Let's begin by understanding the meaning of this sentence. We'll use our understanding of pause points and segment the sentence as shown - understanding and assimilating the meaning of each segment bit by bit!

Sentence Structure

• In a rural area along the border between Oklahoma and Texas,
  • amateur astronomers gather each year
    • to observe the night sky
      • at the Okie-Tex Star Party.

• Like most star parties,
• Okie-Tex takes place in an area with low light pollution,
  • [?] dark skies and ideal stargazing conditions.

• Where [?] varies between:
• ensuring
• this will ensure / ensures / it ensures

Understanding the Meaning

Let's start from the beginning.

The first sentence sets the scene:

• 'In a rural area along the border between Oklahoma and Texas, amateur astronomers gather each year to observe the night sky at the Okie-Tex Star Party.'
  • This tells us about an annual event where astronomers come together to look at stars.

Now the second sentence:

• 'Like most star parties, Okie-Tex takes place in an area with low light pollution,'
  • This is the main statement of the sentence.
  • Subject: Okie-Tex (the star party)
  • Verb: takes place
  • Where: in an area with low light pollution

This is where we have the blank. Let's look at the choices:

• We're choosing between:
  • "ensuring" (an -ing form)
  • "this will ensure" / "ensures" / "it ensures" (forms that include the verb "ensure")

To see what works here, let's read the rest of the sentence and understand what it's saying!

After the blank: 'dark skies and ideal stargazing conditions.'

• This is telling us about the result or benefit of having low light pollution—
  • it leads to dark skies and perfect conditions for stargazing.

What do we notice about the structure here?

We already have a complete main statement:

• 'Okie-Tex takes place in an area with low light pollution'
  • This is a complete thought with its subject and verb.

After the comma, we're adding information about what that low light pollution does—it results in dark skies:

• This is extra descriptive information, not a new main statement.
• When we want to add this kind of consequential or descriptive detail to a complete thought, we use a comma followed by an -ing form.
  • This creates a modifying phrase that describes the result or effect.

If we used "this will ensure," "ensures," or "it ensures," we'd be creating a second complete thought:

• Two complete thoughts can't be joined with just a comma—that would create a comma splice error.

So we need ensuring—it creates a descriptive phrase that flows naturally from the main statement.

The correct answer is A. ensuring.

GRAMMAR CONCEPT APPLIED

Using -ing Forms to Add Descriptive or Consequential Information

When you have a complete statement and want to add information about a result, consequence, or related action without starting a whole new sentence, you can use a comma followed by an -ing form (called a present participle in grammar terms). This creates a modifying phrase that adds detail to your main statement:

Pattern:

• Complete main clause + comma + -ing phrase (describing result/consequence)

Example 1:

• The company invested in new technology, improving efficiency across all departments.
• Main statement: "The company invested in new technology"
• Result/consequence: "improving efficiency" (modifying phrase)

Example 2:

• Scientists discovered a new species in the deep ocean, expanding our understanding of marine biodiversity.
• Main statement: "Scientists discovered a new species in the deep ocean"
• Result/consequence: "expanding our understanding" (modifying phrase)

In this question:

• Main statement: "Okie-Tex takes place in an area with low light pollution"
• Result/consequence: "ensuring dark skies and ideal stargazing conditions"
• The -ing form "ensuring" creates a phrase that describes what the low light pollution accomplishes

Why not a new clause? If you use forms like "this will ensure," "ensures," or "it ensures," you create a second complete thought (independent clause), which cannot be joined to the first complete thought with just a comma. That would be a comma splice—a common grammar error on the SAT.