Final answer:
To clarify the sentence, the pronoun 'she' should be replaced with 'Breanna' to clearly indicate who realized that they were out of flour.
Step-by-step explanation:
The sentence "When Breanna and her sister began baking the cake, she realized they were out of flour." can be made more clear by changing "she" to "Breanna". This correction clarifies who realized that there was no flour. Without this change, it's ambiguous whether "she" refers to Breanna or her sister.