* Suitable/fitting: approprié (masculine) or appropriée (feminine)
* Proper/correct: convenable or correct
* Suitable for a particular purpose: adapté (masculine) or adaptée (feminine)
You can also use convenant (masculine) or convenante (feminine) if you want to emphasize the idea of something being "fitting" or "becoming".
Here are some examples:
* "C'est un cadeau approprié pour un enfant." (This is a suitable gift for a child.)
* "Il est important d'avoir un comportement convenable en public." (It is important to have proper behavior in public.)
* "Ce livre est adapté aux enfants de 5 ans." (This book is suitable for 5-year-old children.)
* "Ce costume est très convenant pour l'occasion." (This suit is very fitting for the occasion.)
The best translation will depend on the specific meaning you want to convey.