NAT (Network Address Translation) allows multiple private IP addresses to share a single public IP when accessing the internet. This conserves public IPs and provides basic security by hiding internal addresses.
B). BGP is a routing protocol.
C). OSPF is a link-state IGP.
D). FHRP provides redundant gateways, not IP sharing.
References (CompTIA Network+ N10-009):
Domain: Networking Concepts - NAT, PAT, private-to-public IP mapping.