The context indicates persistence and determination despite setbacks.
- "Resolute" signifies firm determination. \(⇒\) This is the best fit.
- "Obstinate" implies stubbornness, often negatively.
- "Relentless" suggests persistence but is generally harsher and less suitable.
- "Unwavering" is similar, but "resolute" is the most appropriate single word.
Therefore, the answer is resolute.