"Continue" is an action-word, so it would be a verb.
Answer:
in
Explanation:
The way that the sentence is structured, it would have to be "in".
<span>Once she left the store, my mother realizes she forgot her purse on the counter.
</span>
Left is past tense, but realizes is present tense.
The way to correct the sentence would be:
Once she left<span> the store, my mother </span>realized<span> she forgot her purse on the counter.
</span>
Your answer is
B.The verb tenses shift.