Which is the valid filename assign in MS-DOS?

Ram_$.txt:
This is a valid filename format in MS-DOS.

Explanation:
In MS-DOS, filenames can have up to 8 characters followed by a period (.) and up to 3 characters for the extension. The characters allowed for filenames are letters (A-Z, a-z), digits (0-9), and certain special characters like underscore (_), dollar sign ($), and tilde (~).

• Ram*.txt: This filename is invalid because the asterisk (*) is not allowed in filenames in MS-DOS.
• Ram?.doc: This filename is invalid because the question mark (?) is not allowed in filenames in MS-DOS.
• Ram_$.txt: This filename is valid because it uses the underscore (_) and dollar sign ($) characters, which are allowed in MS-DOS filenames.
• None of above: This option is incorrect because Ram_$.txt is a valid filename format in MS-DOS.

Therefore, the correct answer is Option C: Ram_$.txt.