For the Quiz a question is said to be correctly answered if all the correct proposals are selected and no wrong proposal is selected i.e

For a quiz

{Question
|type="[]"}
+ Option A ( Correct answer ).
- Option B ( Incorrect answer ).
+ Option C ( Correct answer ).
- Option D ( Incorrect answer ).

The user is only awarded correct points if he/she selects both A and D.
The points are not awarded on basis of individual proposals, therefore in the above quiz the score is -1/3 as the answer was not correct.

The reason for this is to avoid the users to select all answers and get score.For the above quiz negative marks are -1 and positive are 3.
So if a user selects all the options then he/she would get 4/6.

I think we shouldn't change the marking scheme.