Correct : b

The Network layer is responsible for routing, so this is correct.

Bit synchronization is actually a function of the Physical layer, not the Data Link layer. The Data Link layer manages framing, error detection, and frame synchronization — not bit timing — so this makes the pair incorrect.

The Transport layer ensures end-to-end process communication, so this is correct.

The Medium Access Control (MAC) sub-layer is responsible for channel sharing and deciding which device can use the channel at a given time, so this is correct.

✅ Final Answer: Option (b) — Data Link Layer and Bit synchronization.

Reason: Bit synchronization is handled by the Physical layer, not the Data Link layer.