Hey ganko,

Yes, it very much important to include Mathematics as one of your subject while counting your best four. For economic hons you need to include Mathematics as one of your best four subject even if  you score less marks in it. University of delhi has made it mandatory for all the candidate to include mathematics if you're opting for ba eco hons.

This action has been taken as Eco (Hons) being a majorly maths driven course, mandatory inclusion of Maths in Best of Four will ensure a better quality of students at Delhi University.

Hope this helps! 

Good luck!