LSI (A): There is no such packet type in OSPF.
HELLO (B): These packets are used for neighbor discovery and maintaining adjacency, not for requesting LSAs.
LSACK (C): Link-State Acknowledgment (LSAck) packets confirm the receipt of LSAs but do not request missing ones.
LSR (D): Link-State Request (LSR) packets are specifically used by routers to request missing LSAs from their neighbors.
Thus, the correct answer is D (LSR).
[Reference: HCIA-Datacom Study Guide, OSPF Packet Types Section., , ]
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