The protocols these variations of the ISDN standard use for communicating between devices and across networks include two bearer or as they are sometimes called "B" channels that are assigned through the ISDN controller for either voice or data, in addition to a data channel used for controlling and ensuring data accuracy. This third channel as a result is sometimes called the "D" channel. How do adapters synchronize their clocks to recognize the first few bits of each frame?

Adapters rely on (a) Using a Prefix, to synchronize their clocks to enable inbound bits to be recognized.

Why do SDLC and HDLC use the terms primary stations and secondary stations?

Because in the context of the OSI Model specifically at Layer 2: Data Link Layer, the primary station and secondary station use a master-slave topology to specifically manage command and response...

The SDLC (Synchronous Data Link Control) protocol is critical for the development of Point to Point, Multipoint, Loop (ring) and Hub-based network topologies. Both CSMA/CD and CSMA/CA network access topologies rely on the role of both SDLC and HDLC to complete integration and communications tasks. As the data link layer of the OSI model is responsible for the hardware-to-software integration of networks, the master-slave approach to accomplishing interoperability is critical, and primary stations and secondary stations are the terms used to define this specific attribute.
What happens when a frame arrives with the BECN bit set to 1?

The indicates that the Backward Explicit Congestion Notification (BECN) has encountered congestion and is reporting back that potential collisions of data packets may occur, specifically in a CSMA/CD-based network.