To ensure reusability of code, the most important aspect to include in line comments is the purpose and/or functionality of the code segment. This helps other developers (or even yourself in the future) understand what the code is intended to do, making it easier to reuse or modify as needed.
While alternative implementations and possible reuse ideas can be beneficial, they are not as critical as clearly explaining the purpose of the code. Additionally, the date of completion may be useful for tracking changes, but it does not directly contribute to the reusability of the code.