Hii

You need to fill the subjects as per the eligibility requirements in the application forms of DU.
This will totally depend on the course you are applying to. Incase the course you apply to has to do with maths then ofcourse you will have to add maths in your best of 4. 
Let me discuss some courses and its eligibility -
BA - for this you will have to select 1 language ( preferably english ) along with other 3 subjects. SO maths will not necessarily be required.
B.COm(Hons) - Aggregate 45% minimum is required in class 12. In best of 4 subjects, maths has to be added this is compulsory along with 1 language and 2 other subjects.
B.Com - 40% aggregate in class 12th. Best of 4 including 1 language and other 3 subjects. Maths is not compulsory

So, this willl depend on the course you are choosing, if maths will be a mandatory requirement, then you will ompulsorily have to fill that in best of four.

Hope this helps
Feel free to revert back incase of any doubt
Thanks!!

Hey, 

it is not compulsory to include the score of maths to calculate the percentage of best 4. You need one language subject And 3 main subjects from- business studies, economics, accountancy, mathematics. You can choose any 4 among these 4. 

hope this is helpful to you.