Since you will be testing a C function, you should use a Target Deployment Port for the C language. Rather than modifying the existing Configuration, you will now create a new one whose base TDP is a TDP for the C language.
To add a new configuration to a project:
Select the menu item Project->Configuration
Note, in the Configurations window that has just appeared, the existence of the Configuration you have been working with up to now.
Click the New... button
In the dropdown list of the New Configuration window, choose either C Visual 6.0 if you have Microsoft Visual C++ 6.0 installed, or, if you are using GNU/native compilers, select the item appropriate for your operating system:
Windows - C Gnu 2.95.3-5 (mingw)
Solaris - C Solaris - SC5.1
Linux - C Linux - Gnu 2.95.2
AIX - C AIX - IBM C Compilers
Do not be concerned if the version of the GNU compiler you have installed does not match the version mentioned for the TDP. The differences are not relevant for this tutorial and thus other versions are supported equally as well.
Click the OK button to close the New Configurations window.
Click the Close button to close the Configurations window.
In the toolbar dropdown list that mentions the current Configuration - named after the C++ TDP you selected at the beginning of the tutorial - select the new Configuration, based on the C TDP you just added to the project. The following is what the box should look like if you're using the Microsoft Visual C/C++ TDPs:
Now the C language TDP will be used by any new node generated via the Activity Wizard.