Final answer:
Comments in a C program can be inserted in any part of the code and are denoted by '//' for single-line, or '/*...*/' for multi-line, and do not affect the program's execution.
Step-by-step explanation:
Comments in a C program can be added in any section: documentation, constant definitions, link sections, or within the actual code body. Comments are denoted by using // for single-line comments, or /*...*/ for multi-line comments. They are used for explaining the functionality of the code, leaving notes for other developers, or for temporarily disabling code without deleting it. Since comments are ignored by the compiler, they do not have any effect on the execution of the program and therefore, can be placed anywhere that is syntactically correct.