However, other abbreviations are possible depending on the context:
* "ch." could be used in some cases, though it might be confused with other words like "chapter".
* "chg." can be used as an abbreviation for "change," so this may not be the most appropriate abbreviation for challenge.
It's important to consider the context and audience when choosing an abbreviation. If you're unsure, it's always best to spell out the word in full.