In the following exhibit, EIGRP is used to advertise routes on both Router A and Router B. Why will packets not route from 172.16.2.0/24 to 172.16.4.0/24?

A. Router A and Router B serial interfaces are not on the same subnet.
B. EIGRP only advertises classful addresses.
C. The serial line is not supported for EIGRP.
D. Auto-summarization is prohibiting proper routing.

D. Auto-summarization is configured by default with EIGRP. Since there are several networks in the 172.16.0.0/22 address range on Router A, they will be summarized. On Router B there is the network 172.16.4.0/24, which would be part of the summarization of the 172.16.0.0/22 network. Since the serial connection is a noncontiguous network, autosummarization will prohibit proper routing operation.