Answer: b. Maybe we should make a new plan, the car is not running.
Step-by-step explanation:
Natural pause during speech is when the speaker has to pause a bit and then continues their dialogue. Commas are the most used to denote natural pauses.
In the above, the best option is B because the speaker came to a natural pause after saying 'plan'. A comma should then be placed there and the rest of the sentence completed.