'월은 한자인데 달은 _____이에요' My guess is that the intended meaning is: "월 is a hanja(한자) word while 달 is a native Koran word". A better way to say it is: '월'은 한자어인데 '달'은 ___이에요. (Strictly speaking, "a hanja-based word" should be expressed as 한자어, not just 한자). The Korean word for "native Korean word" is 순우리말, or 고유어 as a second choice. So we can say: '월'은 한자어인데 '달'은 순우리말이에요. 순우리말 is "순(pure) + 우리말(our word, our tongue/language)", where 순 is a prefix used to emphasize the nativeness. 순 is kind of necessary in this context because if we just say 우리말, the implication would be that 한자 is not part of Korean, which is not true. 우리말 is a common word by which Koreans refer to their language, together with 한국말. Just like 우리나라 (our country), it has the familiar "tribal" ring to it, and it's a Korean native word itself. 고유어 is a hanja word for "native tongue/language". It's used more often for languages of other nations.

Can you give me more clue? :) are there three blocks on the blank? haha. 월은 한달을 가르키는 한자어 인데 5(오)월 6(유)월 이렇게 몇월인지 말할 때 쓰고요, 한달, 두달 이렇게 기간을 나타낼 때 달을 쓴답니다! 그리고 기간을 말할 때는 개월로도 말해요, 1개월, 6개월 등등의 표현도 써요.

I think you're looking for 순우리말 (pure Korean)

'月'은 '한자', '월'은 '한자어', '달'은 '고유어'예요.