Step 1: Understanding the task.
To save a plot as a file in matplotlib, the function \texttt{savefig()} is used. This function saves the current figure to a specified file format (e.g., PNG, PDF, SVG, etc.).
Step 2: Evaluate each option.
- Option (1) plt.export(): There is no function named \texttt{export} in matplotlib for saving figures.
- Option (2) plt.save(): This is not a valid function in matplotlib.
- Option (3) plt.savefig(): This is the correct function to save the plot. It saves the current figure to the specified file.
- Option (4) plt.store(): This is not a valid function in matplotlib.
Step 3: Conclusion.
The function \texttt{savefig()} is the correct way to save a matplotlib plot as a file.
Final Answer:
\[
\boxed{\text{Use plt.savefig() to save a plot as a file in matplotlib.}}
\]