In computer networking, how many layers are defined in the OSI reference model?

Introduction / Context:
The OSI (Open Systems Interconnection) reference model is a conceptual framework used to understand and design network systems. It divides network communication functions into a set of layers, each with specific responsibilities. Knowing the number of layers and their purpose is fundamental in computer networking courses and examinations.

Given Data / Assumptions:

• The question asks specifically about the number of layers in the OSI model.
• The options provide four different numeric values: 13, 11, 9, and 7.
• We assume the learner has basic exposure to network models and terminology.
• No additional calculations are needed; this is primarily a recall concept.

Concept / Approach:
The OSI model is standardized by the International Organization for Standardization. It defines seven distinct layers, from the physical layer at the bottom to the application layer at the top. Each layer provides services to the layer above and uses services from the layer below. Therefore, the correct number is fixed and well known as seven. Any other number in the options represents a distraction or confusion with other models.

Step-by-Step Solution:
Step 1: Recall the OSI layers in order: Physical, Data Link, Network, Transport, Session, Presentation, Application.Step 2: Count these layers and note that there are seven distinct layers.Step 3: Compare this count with the numeric options 13, 11, 9, and 7.Step 4: Observe that the only option that matches the known count of OSI layers is 7.Step 5: Select 7 as the correct answer.

Verification / Alternative check:
If you check networking textbooks, certification guides, or official standards, all of them describe the OSI reference model as having seven layers. The popular mnemonic “Please Do Not Throw Sausage Pizza Away” is often used to remember these seven layer names. No widely accepted variant of the OSI model uses 9, 11, or 13 layers, which confirms that 7 is the only correct option.

Why Other Options Are Wrong:
The numbers 13, 11, and 9 are simply distractors. They may confuse learners who mix up the OSI model with other layered models or who incorrectly remember the count. No standard network architecture model widely used in basic courses describes exactly 9, 11, or 13 layers in the same context as the OSI reference model. Therefore these options are not valid representations of the OSI layer count.

Common Pitfalls:
A frequent error is confusing the OSI model with the simpler TCP or Internet model, which is often described using 4 or 5 layers. Another pitfall is failing to memorize both the names and the number of layers, leading to guesses based on arbitrary numbers. Practicing the standard list of seven layer names usually helps avoid this confusion.

Final Answer:
The correct answer is 7, because the OSI reference model formally defines seven layers for network communication.