D (GRE Key and Sequence Number extensions):The GRE Key field provides session identification for distinguishing between multiple GRE tunnels. The Sequence Number extension allows endpoints to track packet ordering and detect lost or out-of-order packets, enhancing reliability at the tunnel level if required.
Other options explained:
A: Protocol Type identifies payload type; Checksum detects errors but doesn't provide sequencing.
B: GRE Version and Reserved0 fields are not used for session or sequencing.
C: GRE does support optional extensions, contrary to this statement.
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