"I buy spring clothes."
adjectives that do not end in oi, ii, ui, ai are na adjectives. They require na to modify nouns eg. kirei na hana - a pretty flower. Some other examples of na adjectives are shitsurei - rude, murasaki - purple, ooki - big (but there is also ookii which also means big and is an i adjective), hen - strange/weird.
If those are the answers you are putting to this question, they're wrong because they are completely different words with different meanings.
服 - Clothes/clothing
靴 - shoe(s)、帽子 - hat(s)、ズボン - pants/trousers
They aren't interchangeable since they all refer to different things...
Based on the comment section, 春服, 夏服, etc. are all preset phrases. It's kind of like "weekend" vs "end of the month" (you can't say "monthend" but you can say "end of the week"). So since there is no set phrase for, say, 春ズボン, you have to make 春 an adjective, like the English example: 春のズボン. Hope that helped a bit