Final answer:
In operating systems such as Microsoft Windows, the period (.) character separates the file extension and the filename.
Step-by-step explanation:
In operating systems such as Microsoft Windows, the character that separates the file extension and the filename is the period (.).
For example, in a file named 'document.txt', the period separates the 'txt' extension from the 'document' filename.
This character is important because it helps the operating system identify the type of file and determine which program should be used to open it.