Which sentence is most clearly written?

The second sentence is the most clearly written because it presents the information in a coherent manner without unnecessary punctuation interruptions. It maintains a smooth flow, allowing the reader to easily understand that Denise experienced a sudden realization about forgetting to lock the car. This construction effectively conveys the thought as a single, complete idea.

In contrast, the first sentence includes an unnecessary period that creates a fragment, interrupting the flow and clarity of the thought. This disrupts the reader's comprehension since it suggests that the first part ("Denise remembered with a start") is a complete thought, which it is not, leading to confusion about the connection between the two ideas. Clarity in writing is essential for effective communication, and the second sentence achieves this by using correct grammatical structure.