What is the purpose of the NBAR discovery protocol?

What is the purpose of the NBAR discovery protocol?

A.
to build a Packet Description Language Module (PDLM) file to be used in protocol matching

B.
to discover applications and build class maps for data classification

C.
to look into the TCP or UDP payload and classify packets based on the content

D.
to build a database of all application data that passes through the router and queue the data accordingly