You need cái to sound natural. Những cái quần là của bạn. This still sounds a bit off those. Although it sounds off in English too.
Những cái quần nầy là của bạn = These pants are yours (now it sounds natural).
yeah you are right, I remember that when we talk about something specific, we should always use the classifier ^_^ and I didn't know "nầy" existed hehe (I just looked it up), southern variant of này I suppose?
Trousers and pants are the same. Why sometime you only accept one of them as correct answer ?
When I was in England trousers and pants were definitely not the same. Only women wore "pants." But that was 30 years ago.