"Digital recording is better" - without context, this should be accepted without the definite article.
It is now. 
Half the time it feels like the French are just playing practical jokes on nonnatives when coming up with new terms.
"enregistrement" = "registration"
So, obviously "L'enregistrement numèrique" = "digital recording"! :]
Pourquoi pas 'mieux'? Est-ce que qqun peut expliquer la différence entre mieux et meilleur ?
Meilleur = adjective. Mieux = adverb.
Plus bon = meilleur and plus bien = mieux. Bon = good and bien = well.
Numérique should be "digital"
There is some peculiar buglike fault in Duolingo which is that sometimes when you tap on those words, there appears some weird gap between them. Therefore, my answer looked like this:
The digital recording is better.
Rejected. It is just weird.