Step-by-step explanation:
Looks at the 5th sentence:
"Before the first out-of-town trip, she..."
Obviously someone new (a person) has been mentioned in the sentence before. The only sentence that has introduced a female subject is the sentence:
"Luckily, Mrs. Greene, the choir director, understood my nervousness."
This sentence also flows from Sentence 4 to 5 nicely, so this is the answer.