Layering and X.25 basics: identify the one incorrect statement about X.25 layers, OSI layer names, and error handling responsibilities.

Introduction:
Networking exams often test whether you can keep straight the relationships among the OSI model, X.25’s three-layer view (Physical, Data Link, Packet/Network), and which layer performs which function (like error detection). This question asks you to pick the single incorrect statement from a set that otherwise sounds plausible.

Given Data / Assumptions:

• X.25 defines a three-layer architecture: Physical (X.21), Data Link (LAPB), and Network (PLP).
• OSI layers from bottom to top are: Physical (1), Data Link (2), Network (3), Transport (4), Session (5), Presentation (6), Application (7).
• Typical link-layer frames include error-detection fields (e.g., CRC) to catch physical-layer errors.

Concept / Approach:
Check each statement against canonical definitions. Confirm that X.25 indeed references three layers; verify that layer 2 is Data Link; remember that the OSI 4th layer is Transport, not Session; and recall that data-link error detection is explicitly designed to catch bit-level corruption introduced on the physical medium.

Step-by-Step Solution:

Verification / Alternative check:
Any standard OSI table will show Transport at layer 4 and Session at layer 5, confirming that calling the fourth layer “Session” is wrong.

Why Other Options Are Wrong (as the incorrect choice):

• A, B, C are accurate statements and align with textbook definitions.
• E (None of the above) would claim there is no incorrect option, which is false because D is incorrect.

Common Pitfalls:
Mixing up Session and Transport layer names; assuming that “None of the above” is a safe pick when one statement is clearly wrong.

Final Answer:
The fourth OSI layer is Transport, not Session, so the incorrect statement is “The fourth layer … is the session layer.”