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

Include guards are a preprocessor directive used to prevent the same header file from being included multiple times in a program. If a header file is included multiple times, it can cause errors such as redefinition of functions or variables. Therefore, include guards are used to ensure that a header file is included only once, which helps with preventing these errors and improving the overall functionality of the program. The other options are incorrect because they do not address the main purpose of include guards in header files. Option A may seem like a correct choice, but include guards are not primarily used for conditional inclusion of header files, but rather to prevent multiple inclusions. Options C and D are also misleading because while they may be potential side effects of using include guards, they are not the main purpose. Overall, the correct answer is B because preventing multiple inclusions is the primary and most important purpose of include guards.