Read the dictionary entry for the word challenge.
challenge /'CHalenj/ noun
1. a feeling or declaration of disapproval or dissent
2. something that requires thought and skill for resolution
challenge /'CHalenj/ verb
3. to demand proof of the truth or rightness of
4. to invite (someone) to take part in a contest or to perform a feat
Which meaning best matches the way the word challenge is used in line 23?