A). a network that uses tunnels to connect two areas - This is not the standard definition of a transit network in OSPF. While tunnels can be used in OSPF for various purposes (e.g., OSPF virtual links), they are not specifically what defines a transit network.
B). a special network that connects two different areas - While an OSPF network might connect two areas, particularly if it's an Area Border Router (ABR), this doesn't define what a transit network is. Any OSPF-enabled network segment where routers form adjacencies and forward data can be a transit network, irrespective of areas.
D). a network that connects to a different routing protocol - This is describing a boundary where OSPF interfaces with another routing protocol, typically managed using redistribution. This isn't what defines a transit network in OSPF.