Hey,

For calculating the best four for bsc chem hons in du you have to include one language subject either it be English or hindi and chemistry necessarily and any two core subjects that can be physics, maths, biology, computer science, economics any of the subjects as mentioned in list A of du. If you do not include any of the subjects from that list you'll face a deduction of 2.5% from your best four percentage .

Also, The overall percentage in Physics, Chemistry and Mathematics should be 55% and  One compulsory language should be 50%.

Good luck !