Final answer:
The correct usage of quotation marks for the sentence in question is option A-, "I don't know," , where the punctuation correctly falls within the quotation marks, in accordance with the rules of American English grammar.
Step-by-step explanation:
The question requires selecting the correct usage of quotation marks within a given sentence. The correct answer is A, "I don't know," she said. "I'm not comfortable buying such a cheap car." The other options are incorrect because they either place the comma outside the quotation marks or fail to include appropriate punctuation to separate the spoken words from the narrative text.
In the context of proper English grammar and punctuation, quotation marks should enclose the exact words spoken by a person, and the associated commas and periods should be placed within the quotation marks in American English.