Explanation
Correct answer: A.
Choice A is the best answer. The convention being tested is the use of verb forms within a sentence. The nonfinite present participle "ensuring" is correctly used to form a supplementary element that modifies the main clause "Okie-Tex takes place in an area with low light pollution," describing the viewing conditions of the star party.
Why the other choices are wrong
Choice B
Choice B is incorrect because it results in a comma splice. The pronoun "this" and the finite future tense verb "will ensure" would add an additional main clause to this sentence, and the comma after "pollution" can’t be used in this way to join two main clauses.
Choice C
Choice C is incorrect because it results in an ungrammatical sentence. The finite present tense verb "ensures" can’t be used in this way to form a supplementary element to modify the main clause.
Choice D
Choice D is incorrect because it results in a comma splice. The pronoun "it" and the finite present tense verb "ensures" would add an additional main clause to this sentence, and the comma after "pollution" can’t be used in this way to join two main clauses.