There are only three articles in the English language. One is definite, and two are indefinite. "The" is a definite article, while "a" and "an" are indefinite articles. To answer your question, you need to find the sentence that correctly uses "a" and "an". "A" is used if the word following it begins with a consonant, while "an" is used if the word following it begins with a vowel. Looking at the options provided in your particular problem, you can see that answer choice a is the only one that follows all of the above rules, making it the correct answer.