The second one I'm pretty sure
You need to be more specific when asking questions like this. First of all, you should copy the second paragraph of whatever you are reading. After doing that, you will be able to get a suitable answer for your question.
Answer: I need more details
Explanation:
1) b and a - A sparse crowd is one that is small and scattered.
2) c and b - To petrify someone is to terrify that person.
3) a and b - An intense pain is one that lasts for a long time.
4) a and b - A prone figure is one that is lying face down.
