On the question:
Why wouldn't "cómo está" be a correct answer? Wouldn't it be formal when speaking to a parent?
it would be very unusual (in Spain) to speak with that formality to a parent, despite them being elderly. It used to be a custom in rural areas in an old fashion way in Spain years ago but it is not nowadays. Very few people maintain that level of formality now.
I have heard a spanish-speaker use 'está' with their madre. I have heard otherwise, as well. Mexican usage, all, as far as I know. It might have something to do with context/some mood of respectfulness. I am not an expert on this, I have just noticed it a small number of times.
Sign in to submit your answer
Don't have an account yet? Join today
Test your Spanish to the CEFR standard