Step 1: Understanding the Python syntax for defining a function.
In Python, the keyword `def` is used to define a function. The syntax is:
\[
\texttt{def function_name():}
\]
This is followed by the function's code block.
Step 2: Evaluating the options.
- (A) def: Correct. In Python, `def` is the keyword used to define functions.
- (B) function: Incorrect. `function` is not a valid keyword in Python to define a function.
- (C) func: Incorrect. `func` is not a Python keyword used for defining functions.
- (D) define: Incorrect. `define` is not a valid keyword in Python to define a function.
Step 3: Conclusion.
The correct answer is (A) `def`, as it is the correct keyword used to define functions in Python.
Final Answer:} def.