Students whose grades are low can sit for the exam again.
Explanation:
We use whose to introduce the relative clause when we are referring to people or things. In this case, the relative clause talks about grades, which is a thing that is why we use whose. We can replace whose with the possessive pronoun their.
In a sentence, the relative clause functions as an adjective because it is modifying the noun just like an adjective does. In this sentence, the relative clause -whose grades are low- is affecting the noun students.
