"Las niñas podrán escribir cartas."

They are actually different tenses in spanish. "Podrán" means "in the future they will be able to", whereas "podrían" means "could" or "would be able to". Subtle difference, especially when this doesn't really exist in English.

I find with Duolingo lessons it is safer to go with the more 'literal' translation when there are multiple options.

For example I got through the ir + a lessons by always saying is going to instead of will — even though technically both would work, about half the time my will answers were marked as incorrect.