Depending on the meaning of "problem":
* Solution: This is the most direct opposite if "problem" refers to a challenge or issue that needs to be resolved.
* Opportunity: If "problem" implies a difficulty, "opportunity" suggests a chance for growth or improvement.
* Success: If "problem" implies failure or setback, "success" is the opposite.
* Advantage: If "problem" refers to a disadvantage, "advantage" is the opposite.
Other related words:
* Benefit: Opposite of a problem that causes harm or disadvantage.
* Asset: Opposite of a problem that is a liability.
* Strength: Opposite of a problem that is a weakness.
Ultimately, the best opposite depends on the specific context and what you want to emphasize.