General Noun:
* Challenge - This is the most direct noun form of the verb "to challenge."
Specific Nouns:
* Obstacle - This refers to something that blocks or hinders progress.
* Difficulty - This refers to something that is hard or demanding.
* Test - This refers to something that assesses or proves ability.
* Trial - This refers to a difficult experience or situation.
The best noun to use depends on the specific context of the sentence.
For example:
* "The mountain climber faced a challenge."
* "She overcame many obstacles in her life."
* "The exam was a difficulty for many students."
* "He passed the test with flying colors."
* "The entire project was a trial for the team."