Final answer:
The correct option is B: The flash mob startled us, but we were too enthralled to leave.
Step-by-step explanation:
The correct sentence that combines the two given sentences into a compound sentence is option B: The flash mob startled us, but we were too enthralled to leave. In a compound sentence, two independent clauses are joined by a coordinating conjunction (such as 'but') or a semicolon. In this case, the independent clauses are 'The flash mob startled us' and 'we were too enthralled to leave.' The coordinating conjunction 'but' connects these two clauses to form a compound sentence.
Learn more about Combining sentences into a compound sentence