Formal:
* Resolved: This implies a firm and determined decision.
* Decided: This indicates a clear choice has been made.
* Determined: This suggests a strong will and commitment to the decision.
Informal:
* Settled: This implies a sense of finality and closure.
* Made up his mind: This is a straightforward and direct phrase.
* Committed: This emphasizes a sense of dedication and allegiance.
* Decided to: This is another direct way to express a decision.
Other options:
* Convinced: This implies a belief or understanding that influenced the decision.
* Certain: This emphasizes confidence in the choice.
* Firm: This suggests a strong and unyielding resolve.
The best word to use will depend on the specific context and the desired tone of your writing.