@Bhawash

In this context, you can say 你很漂亮 or 你是一个很漂亮的姑娘. 你是漂亮的 is also grammatically correct but sounds a bit weird in oral Chinese. 是 is often used with 的 at the end of the sentence, to indicate category, characteristic, etc.

i.e. 书是红色的； 我是快乐的.