If you're talking about a set of keys:
* They - This is the most common and general pronoun. Example: "I found the keys, but I don't know if they're mine."
* Those - This is used if you're referring to specific keys that have been mentioned before. Example: "I'm looking for those keys, the ones with the blue tag."
If you're talking about a key individually:
* It - Example: "I need a key to unlock this door, but I can't find it anywhere."
If you're using keys metaphorically:
* They can still work. Example: "The keys to success are hard work and dedication."
Ultimately, the best pronoun to use depends on the specific situation and how you want to express your meaning.