Hello

It is not necessary for you to appear in improvement exam in all the four subjects. It is your choice. If you want to give exam for all the four subjects again then surely you can, if it improves yourc hances of securing a seat in DU but it is not compulsory. At the end of the day it depends on the aggregate, if by giving only one exam your aggregate of the four subject crosses the cutoff required by particular colleges then you just  haave to give one improvement exam. For more details you can click on https://university.careers360.com/articles/du-admission