Step 1: Understanding the Concept:
In computer programming, a function is a block of organized, reusable code that is used to perform a single, related action.
To create a function, a language requires a specific "reserved word" or keyword that tells the interpreter or compiler that a new block of code is being defined.
Step 2: Detailed Explanation:
Python is known for its concise and "human-readable" syntax.
In Python, functions are defined using the reserved keyword def.
The keyword \texttt{def} stands for "define".
The general syntax of a function definition in Python is as follows:
\[ \text{def function\_name(parameters):} \]
This line is followed by an indented block of code which constitutes the function's body.
Let's evaluate the alternative options provided in the question:
Option (A) function: This keyword is used in languages like JavaScript, PHP, and Swift to declare functions, but it is not a valid keyword in Python.
Option (B) define: While "def" is short for "define," the full word is not a reserved keyword in Python. However, "define" is often used in C/C++ preprocessors (as \#define).
Option (D) fun: This keyword is used in modern functional languages like Kotlin or Rust (which uses \texttt{fn}), but it is incorrect in the context of Python.
Python's design philosophy (the Zen of Python) emphasizes having "only one obvious way to do something."
Therefore, \texttt{def} is the only keyword used for standard function definitions.
It is also important to remember that Python function definitions must always end with a colon (:) and the following code must be consistently indented.
Step 3: Final Answer:
Therefore, the correct keyword to define a function in Python is \texttt{def}, which corresponds to option (C).