Translation:He has adapted them to be the best.
Il les a adaptées pour être les meilleures. i do not understand why does adaptées have to feminine
There needs to be an agreement between number/gender of "adapté" and number/gender of "meilleur" since they refer to the same thing.
In this example, "adapté" agrees in number/gender of the first "les" (since it appears before the auxiliary verb) and is apparently feminine, but you should be able to have "Il les a adaptés pour être les meilleurs." if "les" was referring to something masculine - without context though, I don't think there is any way to know.
If this was a listening sentence, I think both should be accepted, since they should be pronounced the same, but just keep in mind they (adapté, meilleurs) should be either both feminine, or both masculine.