Peter39;s work is getting worse and worse.Unless it.....,he39;ll fail his exams

There are 3 types of conditional sentences. First type talks  about something that can realistically happen in present and/or future tenses. Therefore the given example is type I. Main part should be in Present simple and the one describing possible outcome in Future simple. 

The correct answer is C. Unless it improves, hell fail his exams in the summer. 

We cannot say unless it doesnt improve because double negative is not allowed in English