Peter's work is getting worse and worse. Unless it ..., he'll fail his exams in the summer. A) doesn't improve B) improves C) will improve D) improved E) won't improve

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, he'll fail his exams in the summer.

We cannot say "unless it doesn't improve" because double negative is not allowed in English