When using extended system ID in 802.1d, how many bits are reserved for this field?

A.
6

B.
8

C.
10

D.
12

Explanation:
The priority value is divided into 4 bit priority and 12 bit extended system id. This extended system
id is usually equal to the VLAN id. The idea behind it is to conserve MAC addresses – PVST+
requires a different bridge id per VLAN, and this would mean different MACs — unless you ensure
that the priorities are different. By re-mapping some of the priority bits, this is ensured and all
VLANs can use the same MAC address.
http://www.informit.com/library/content.aspx?b=CCIE_Practical_Studies_II&seqNum=13