Which of the following defines a syntax error?

A syntax error is specifically defined as an error in the source code that violates the syntax rules of the programming language being used. These rules dictate how code must be structured, including the correct use of symbols, keywords, and command formats. When a syntax error occurs, the compiler or interpreter cannot process the code and will typically output an error message, indicating the location and nature of the problem.

In the context of programming, this is an essential concept because it affects the correct compilation or interpretation of code, making it impossible for the program to run until the error is corrected. Syntax errors can arise from misspellings, incorrect punctuation, or incorrect arrangement of commands. Understanding this helps developers strive to write clean, correctly structured code to avoid such issues.

The other options provided do not encapsulate the definition of a syntax error. Success in executing the program with errors pertains to runtime issues rather than syntax. A comment is simply a part of the code meant for clarification and does not constitute an error. Lastly, optimization problems relate to improving code performance or efficiency, which again do not relate to syntax violations.