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The function that is used to create a portable file path string is indeed the one that combines different components of a file path into a single, correct format that works across various operating systems. This is especially important because different operating systems use different path separators; for instance, Unix-based systems like Linux and macOS use the forward slash (/), while Windows uses the backslash (\).

Using the specified function from the `os.path` module, it takes multiple string arguments representing different parts of the file path and joins them together with the appropriate separator for the operating system on which the code is being executed. This makes it easier for developers to write code that is platform-independent. By using this function, you avoid potential errors that arise when manually concatenating file paths with hard-coded separators.

In contrast, the other functions mentioned do not serve the purpose of creating portable file paths. For example, checking if a path is a mount point or merging paths without proper consideration for the operating system's format would not provide the desired portability and reliability that the correct function offers. Thus, the choice made is the right one to ensure that file paths are correctly formed in a way that can be used regardless of the operating system being employed.