Up until now I thought all words in English ending in sion became cion in Spanish
Ha yes, I forgot about television. If you'd asked me I would've said that extension was extencion. A reminder to beware of false friends
more subtle than that: something that ended in -tio in Latin will convert to -cion in Spanish -- which is either helpful on not, depending how much Latin a person knows. But -ion will always be feminine, which is nice.
I'm not sure but I think that only Latin words ending in -tio (in English -tion) ends with -cion in Spanish.
Prolongación is correct, but elongation = alargamiento, while enlargement (which is the false friend of alargamiento) = ampliación. It's interesting how these false friends work in terms of having interrelated meanings.
I accidentally put "the etention." If it's a word please tell me what it means. (It counted "etention" as correct.)
It is not a word, as far as I know. It probably gave you credit and said you had a typo.