A. To join independent clauses when the second clause explains the first.
The topic is the general subject of a paragraph or essay. Topics are simple and are described with just a word or a phrase. The main idea is a complete sentence; it includes the topic and what the author wants to say about it.
<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.