まきさんはなんねんせいですか？ --> In which / what grade Maki-san is? (The example answer is in the second grade now.) かよってるだいがくはどこですか？ --> What is the university which you go to? せんこうはなんですか？ --> What (subject) is your major? (*せんこうするは --> せんこうは) けんさんは２３さいです。--> Ken-san is 23 years old. けんさんはよんねんせいです。--> Ken-san is in the fourth grade. (*よんねんせい --> よねんせい) けんさんのだいがくはにほんだいがくです。--> The university which Ken-san goes to is Nihon University. けんさんのせんこうするはにほんごです。--> Ken-san's major is Japanese / Ken-san is a Japanese major. I hope this was helpful and if my English isn't correct, I'd like you to know the correct ways to say.

せんこうする→せんこう The rest is right.