on this line c= a,b; the assignment operator has higher precedence than the , operator so assignment is done first so c = 5;
on this line c=(a,b); the parenthesis make the comma operator evaluate first the comma operator returns its right hand operand so (a,b) evaluates to 6 so then you have c = 6;
Did they give any reason why they were expecting you to know such pointless detail, that could be trivially looked up if it ever actually became an issue?