It has to be B because it was the day she was taking the test and while she looked at the questions on the test that the questions were impossible to do which gave that part of the plot a little bit of suspense since her grade depended on the exam it can make people wonder if she would fail the exam or not.
Explanation: The rule for indefinite article usage is as follows: Use before nouns (or adjectives) that start with a consonant sound. Use before nouns (or adjectives) that start with a vowel sound.
