"I have clothes."
The Nominative form is either "ubranie", which is neuter (but that is like one set of clothes, for example the clothes that you are wearing at the moment), or "ubrania", when you just use plural.
"ubranię" would work if the basic singular form was "ubrania", but it's not.
"ciuchy" is a completely colloquial word. We generally don't accept colloquialisms.
What about "odzież"? It was marked as a wrong answer. Shouldn't it be also considered the right answer?