Underscore ( _ ) symbol can be used in variable name.
In C programming, a variable name can consist of a combination of letters both in lower case and upper case, digits, or integer numbers, and only one special symbol underscore (_). A variable name may begin with a letter or an underscore. Other than underscore(_), no other special symbol can be used in a variable name.
Looking at the options, we can clearly eliminate A, B, and C. So the correct choice is option D.
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