Compare each of the three others' score to Farhan's.

Bristy got the highest marks and Mursalin got the lowest marks.

Let $B,F,Z,S$ be the scores of Bristy, Farhan, Muaz and Mursalin respectively.

Muaz determined that two of them got the same score by only knowing his own and Farhan's score, so $F=Z$.

Bristy determined that she didn't get the lowest score by only seeing Farhan's score, so $F<B$.

Mursalin determined that he didn't get the highest score by only seeing Farhan's score, so $F>S$.

\[\therefore B>F=Z>S\]

So, Bristy got the highest marks and Mursalin got the lowest marks.