For user-defined header files in C and C++, which file extension is most commonly used in source code examples such as #include "myheader.h"?

Introduction / Context:
Header files in C and C++ are used to declare functions, classes, and other interfaces that can be shared across multiple source files. The naming convention for header files, especially user-defined ones, often includes a particular file extension. This question asks you to identify the most commonly used extension when you see examples like #include "myheader.h" in textbooks and code samples.

Given Data / Assumptions:

• We are dealing with user-created header files, not necessarily system headers.
• #include "file" syntax is used, which usually refers to a local or project header file.
• We are interested in the conventional extension used in many C and C++ examples.
• Alternative extensions such as .hpp or .hh exist, but the question focuses on the most common classic form.

Concept / Approach:
Historically, C programs have used the .h extension for header files, and this convention carried over into C++ as well. While C++ projects sometimes use .hpp or similar extensions to distinguish C++ headers from pure C headers, the simple .h suffix remains very widespread, especially in teaching material and mixed language code bases. When you see tutorial examples like #include "myheader.h", the .h extension is used to mark user-defined header files that contain declarations to be shared. Therefore, among the options given, .h is the most appropriate answer.

Step-by-Step Solution:
Step 1: Recall that standard C headers are things like stdio.h and math.h, which clearly use the .h extension. Step 2: Note that many user-defined headers in tutorials are named with the same convention, for example mylib.h or utilities.h. Step 3: Recognise that .cpp is commonly used for C++ source files, not header files. Step 4: Observe that .hf is not a standard or widely used extension in C or C++ projects. Step 5: Choose .h as the conventional extension for user-defined headers in classic examples.

Verification / Alternative check:
You can quickly verify this by browsing existing open source C and C++ projects. You will see many header files ending in .h and source files ending in .c or .cpp. Some modern C++ projects indeed use .hpp, but that extension is not among the options in this question. Additionally, most textbooks and exam questions that show simple #include "header" syntax use .h, confirming it as the most recognisable answer here.

Why Other Options Are Wrong:
Option cpp: This extension is associated with C++ source files that are compiled directly, not header files included by other files. Option hf: This is not a standard or commonly adopted extension in C or C++ development. Option none of these: This is incorrect because .h is widely used and clearly fits the example given in the question.

Common Pitfalls:
Learners sometimes think that any file used in a project can have any extension, which is technically possible but breaks conventions and tooling expectations. Another pitfall is to confuse .cpp and .h, especially when organising project structures. Following conventional naming patterns, such as pairing file.cpp with file.h, makes it easier for other developers, build systems, and editors to understand and work with the code. For exams, remembering that .h is the classic header extension is usually sufficient.

Final Answer:
The most commonly used extension for user-defined header files in examples like #include "myheader.h" is h.