Formal:
* Inevitably: This implies something is unavoidable and must happen.
* Unalterably: This means that something cannot be changed.
* Irrevocably: This implies a finality and that there is no turning back.
* Exclusively: This emphasizes that there is only one option.
Informal:
* Only: This is a simple and direct way to say that there is no other choice.
* Absolutely: This adds emphasis to the statement that there is no other way.
* Solely: This emphasizes that something is the only option.
* Necessarily: This implies that something is required and there is no alternative.
Figurative:
* The only path: This suggests that there is a clear and defined route to be taken.
* The only way forward: This implies that progress can only be made by following a specific course.
The best word to use will depend on the specific context and the intended meaning.