Hi! I added some sentences. か　is used for asking questions (formal style/です・ます) And when you want to speak in casual style, thenか isn’t used. ＊ じしょを　つかいますか？（verb）→じしょ（を）つかう？ ＊ それは　おいしいですか？(adjective)→それ（は）おいしい？ ＊ かれは　学生ですか？(noun)→かれ（は）学生？ (past/negative are same.) ～んです is also formal style because it uses です In this case when you want to speak in casual style, の　is used instead of んですか ＊ じしょを　つかうんですか？→じしょ（を）つかうの？ ＊ それは　おいしいんですか？それ（は）おいしいの？ ＊ かれは　学生なんですか？→かれ（は）学生なの？ (past/negative are same.) Besides it, sometimes men add か　with these casual style sentences. ＊ じしょ、つかうか？/ じしょ　つかうのか？ ＊ それ　おいしいか？/ それ　おいしいのか？ ＊ かれ　学生か？/ かれ　学生なのか？

If I would answer this question simply.... １．　か　is always the marker of "this is a question!!", which always appears after 丁寧体（ていねいたい　polite form）. Ex. まいにち、にほんごを　べんきょうしますか。 　　いま、どこに　いますか。 Then, if you change sentences into 普通体(ふつうたい、normal form), your sentences would be... まいにち　べんきょうする　？ いま、どこにいる？ ２．　But sometimes sentences in normal form sound a little strong, direct, or too frank. So, some people (mostly woman, kid but sometimes men too, without any connotation) would like to put の　after the sentences. It is done to show the friendliness, sympathy etc. まいにち　べんきょうするの？ いま　どこに　いるの？ Note: の basically has a some function with んです　（みんなの日本語Lec26）. But after normal form, it can sound differently from んです. So when you use の、it is better to be careful to the intonation of your speech. I hope I explained in a good way !! Very very good and difficult question !

I think 'か？' is correct grammar of japanese lang. but mainly women use 'の？' for question ( answer Yes ) with conversation.