Mastering C++: A Comprehensive Quiz Based on 'Thinking in C++'

Side effects occur when a function or operation modifies a variable or other condition outside of its own scope. If macros are evaluated every time they are used, this means that the arguments passed into the macro will also be evaluated every time, potentially resulting in unexpected side effects. Options A, B, and D are incorrect because they do not address the issue of side effects. Option A is incorrect because the return value of macros can still be used, even if arguments are evaluated every time. Option B is incorrect because the execution time of the macro may not necessarily increase, depending on the arguments passed in. Option D is incorrect because macros can still accept multiple arguments, even if they are evaluated every time. Therefore, the correct answer is C, as it explains the potential consequence of having arguments in macros that are evaluated every time they are used.