Assume that source S and destination D are connected through two intermediate routers labeled R. Determine how many times each packet has to visit the network layer and the data link layer during a transmission from S to D.

During packet transmission across the network:

1. Source Host (S): The packet originates at higher layers and traverses down through the Network Layer (1st visit) and then the Data Link Layer (1st visit) to the physical medium.

2. First Intermediate Router (R1): The packet moves up through the Data Link Layer (2nd visit) to the Network Layer (2nd visit) for routing, then down through the Data Link Layer (3rd visit) to the next link.

3. Second Intermediate Router (R2): The packet moves up through the Data Link Layer (4th visit) to the Network Layer (3rd visit), then down through the Data Link Layer (5th visit) to the final link.

4. Destination Host (D): The packet moves up through the Data Link Layer (6th visit) to the Network Layer (4th visit) and finally to the upper application layers.

Total visits:

• Network layer: $1 + 1 + 1 + 1 = 4$ times

• Data link layer: $1 + 2 + 2 + 1 = 6$ times

Therefore, (C) is the correct option.