Well the "correct answer" given is "I am paying the interest", so it would seem to be fine (given that German verbs use the same form for "I x" as "I am xing"). I'm not sure about the article, but all that does in English is make it a more specific interest rather than just interest in general.
I wrote "I pay the interests" and I got wrong because in this case I should have written interest (singular). I find this surprising, as I thought there were, by definition, a lot of "interests" to be paid to a Bank. Or is this some specific rule of English (it's not my native language)? Otherwise, I feel my answer should have been accepted, since Zinsen is itself the plural of Zins.
Actually my question was why you use the plural in German (die Zinsen) for "the interest" instead of "der Zins" which also exists as a word. I am not a German native speaker and I hope you are. Does "der Zins" have other meaning in Germany (I don't care about Austria and Switzerland).
der Zins is the singular, die Zinsen the plural.
"Ich bezahle die Zinsen" = "I'm paying the interest" - but I'd like to know why the German sentence uses the plural form. Is it implying that "I" am paying the different interest rates on my mortgage, credit card and bank overdraft, or something else?