B. Donna, who is recalling something Mom said as they traveled
Explanation:
The narrator of a story uses first person speech. First person includes words such as "I" and "me" and "we."
The narrator cannot be Mom because she is referred to in the third person.
You can see that Mom is talking to Donna. Therefore, we know that both Mom and Donna are in the car during this conversation. And because we see the word "we" is used, we know it must be either Mom or Donna. We already eliminated the option of it being Mom, so the answer is Donna.