Which of these is a valid differentiated services PHB?

A.
Guaranteed PHB

B.
Class-Selector PHB

C.
Reserved Forwarding PHB

D.
Discard Eligible PHB

E.
Priority PHB

Explanation:
The Per-Hop Behavior is determined by the differentiated services (DS) field of the IPv4 header or IPv6 header. The DS field consists of a 6-bit differentiated services code point (DSCP).[2] Explicit Congestion Notification occupies the least-significant 2 bits.[3][4][5]
In theory, a network could have up to 64 (i.e. 26) different traffic classes using different markings in the DSCP. The DiffServ RFCs recommend, but do not require, certain encodings. This gives a network operator great flexibility in defining traffic classes. In practice, however, most networks use the following commonly-defined Per-Hop Behaviors:
Default PHB (Per hop behavior)which is typically best-effort traffic
Expedited Forwarding (EF) PHBdedicated to low-loss, low-latency traffic
Assured Forwarding (AF) PHBgives assurance of delivery under prescribed conditions
Class Selector PHBswhich maintain backward compatibility with the IP Precedence field.