Why link-state ACK is used in OSPF? What is the role of re-transmission timeout here?

Retransmission timeout means if a LSA is not acknowledged after how much time we have to retransmit it,

Ack is used to tell the sending router LSA has been received.

OSPF is a reliable protocol and generates Ack packet for DBD, LSR, LSU.

If any packet mismatch or missed than router again request for it using LSR and other routers will re-transmit the packet again.

LSAck ensures the successful delivery of the LSU.