"pero prefiero croquetas de jamón"

Question

Is it possible to omit "Las" or use "unas" instead? Thank you.

Answer

Hola lliass,
I checked this exercise and there was a typo. We shouldn't have used the article "una" with tortilla. You can use it in English but not in Spanish in that sentence. The right way is:
Su plato favorito es la tortilla pero yo prefiero las croquetas de jamón.
You can also omit the articles in Spanish, saying:
Su plato favorito es tortilla pero yo prefiero croquetas de jamón.
Using "una" and "unas" would be correct grammatically but we don't normally use them when we are talking about preferences in general.
That has already been corrected in the exercise.
Saludos
Inma