Answer:
b. a gentle murmur.
Step-by-step explanation:
The definition of to lull includes: "to cause someone to feel calm or to feel that they want to sleep" and "calm or send to sleep, typically with soothing sounds or movements". There was sound, so it wasn't quiet. It was a calm sound, though, so we can discard waterfalls and rapids. We are left with a gentle murmur.