When protocol data is encapsulated as it moves down the TCP/IP or OSI stack for transmission, what is the correct order of the encapsulation units from the application data down to the physical medium?

Introduction / Context:
Networking models such as the OSI and TCP/IP stacks describe how data is encapsulated as it travels from an application down to the network interface. At each layer, headers (and sometimes trailers) are added, creating a specific type of Protocol Data Unit (PDU). Understanding the correct encapsulation order is a foundational concept for Cisco certifications and helps in troubleshooting issues related to headers and addressing.

Given Data / Assumptions:

We are considering a typical TCP/IP stack mapped to the OSI model for a unicast transmission.
Application layer sends user data down to the transport layer (for example, TCP).
The question mentions data, segment, packet, frame and bit, which correspond roughly to different layers PDUs.

Concept / Approach:
At the top, the application, presentation and session layers can be thought of as working with application data. The transport layer (TCP or UDP) encapsulates this data into segments (TCP) or datagrams. The network layer (IP) encapsulates segments into packets (also called IP datagrams). The data link layer encapsulates packets into frames, adding MAC addresses and other link specific information. Finally, the physical layer converts frames into a stream of bits on the wire or other medium.

Step-by-Step Solution:
Start with application data at the upper layers; this is simply referred to as "data". The transport layer (for example TCP) takes this data and forms a "segment" by adding transport headers such as source and destination ports. The network layer (IP) takes the transport segment and encapsulates it inside an IP "packet" by adding IP addressing and related fields. The data link layer wraps the IP packet into a "frame", adding MAC addresses and frame check sequence, suitable for the local network technology such as Ethernet. The physical layer finally transmits the frame as a stream of "bits" over copper, fiber or wireless medium.

Verification / Alternative check:
Many textbooks summarize this as the mnemonic "data → segment → packet → frame → bits" when going from top to bottom, and the reverse order when going from bottom to top during decapsulation. This mapping can also be cross checked with OSI layers: application data (layers 5–7), segment (layer 4), packet (layer 3), frame (layer 2) and bits (layer 1). The option that matches this order exactly is option c.

Why Other Options Are Wrong:
Option a mixes the order by placing frame before packet, which does not reflect the real encapsulation sequence.
Option b incorrectly starts with segment before data and jumbles the order of data and packet.
Option d swaps packet and frame, which is not correct according to the OSI and TCP/IP models.
Option e similarly has data appear after packet, which does not align with the normal top down process.

Common Pitfalls:
Learners sometimes confuse packet and frame, using the terms interchangeably. Remember that "packet" is associated with the network layer (IP) and "frame" with the data link layer (Ethernet, PPP and so on). Another pitfall is to forget that encapsulation adds headers (and trailers) at each layer; if the order is misunderstood, interpreting protocol analyzers like Wireshark becomes more difficult.

Final Answer:
The correct encapsulation order from application down to the physical medium is Data → Segment → Packet → Frame → Bit.