is for C source code editing, completions, formatting, compiling, folding, searching, executing, syntax checking, debugging, and other modern IDE features that Emacs supports.
Use the c tag for C and Related Modes, which include C, C++, Objective-C, Java, CORBA IDL, Pike and AWK modes.
Emacs also provides basic support of c programming through the cc-mode.
There are separate tags for c++ and c++-mode as well.