When variable length subnet masking is used, what does the term route aggregation describe?

A.
calculating the total number of available host addresses in the AS

B.
combining routes to multiple networks into one supernet

C.
reducing the number of unusable addresses by creating many subnets from one supernet

D.
reclaiming unused address space by changing the subnet size

Explanation:
Route aggregration is an effort to route smaller prefixes via an aggregated larger prefix (supernetting). The advantage is obvious: Many /24 networks, for example, could be aggregated to larger networks like /23, /22 or even bigger prefixes.