Threads: When the request for the background work comes in, the web role starts as many threads as necessary (or queues the individual work items to the thread pool). In this option, we would configure a larger instance during the heavy workload, because these threads could require a lot of memory.
Incorrect:
Not C, not D: SLA would require multiple role instances.
References: https://stackoverflow.com/questions/12891678/orchestrating-a-windows- azure-web-role-to-cope-with-occasional-high-workload