Review the original item:
Susan ducked when she saw Professor Hayden walk into Tito's Taco
Palace, unfortunately, a burrito was not big
enough to hide behind, so poor Susan had to explain why she had missed class
again.
You chose the correct way to fix the error:
Susan ducked when she saw Professor Hayden walk into Tito's Taco
Palace. Unfortunately, a burrito was not big
enough to hide behind, so poor Susan had to explain why she had missed class
again.
Susan ducked ... begins a complete sentence. Unfortunately, a burrito was not big enough ... starts the next sentence.
You are always correct when you use a period and a capital letter between the two sentences that create a comma splice.