General:
* Obviously
* Plainly
* Evidently
* Explicitly
* Distinctly
* Unambiguously
* Intelligibly
* Understandably
Emphasis on visibility:
* Visibly
* Plainly
* Openly
Emphasis on understanding:
* Explicitly
* Understandably
* Intelligibly
* Cogently
Emphasis on certainty:
* Definitely
* Absolutely
* Positively
The best synonym depends on the specific meaning you want to convey.