Final answer:
In OSPF, the DR and BDR elections are performed in Broadcast and NBMA networks to optimize network resources and minimize the amount of OSPF traffic. These elections help network efficiency by reducing the number of LSA exchanges in environments where multiple routers connect to the same physical medium.
Step-by-step explanation:
The OSPF network where DR (Designated Router) and BDR (Backup Designated Router) elections are performed is a Broadcast Network (Choice B) and a Non-Broadcast Multi-Access (NBMA) Network (Choice C). OSPF protocol uses the concept of DR and BDR in multi-access networks to minimize the number of LSA exchanges and to optimize the network resources. In these environments, multiple routers can connect to the same physical medium.
In a Broadcast Network, such as a typical Ethernet LAN, all devices can receive a packet that any one of the devices sends. In this scenario, there might be multiple routers on the same network segment. To reduce the amount of OSPF traffic, one router is elected as the Designated Router and another as the Backup Designated Router.
In an NBMA Network, which includes Frame Relay or ATM networks, devices are interconnected in such a way that not all devices can reach each other directly. Due to the potential complexity of OSPF adjacencies in such networks, DR and BDR elections are critical to reduce the number of required adjacencies.
Point-to-Point Network (Choice A) and Point-to-Multipoint Network (Choice D) do not perform DR and BDR elections because these network types do not have the 'multi-access' characteristic. In Point-to-Point networks, there are only two devices communicating with each other, and in Point-to-Multipoint networks, one device is connected to multiple destinations without expecting full mesh interconnectivity between those destinations.