Hi

To calculate best of four for maths honors, you need to calculate marks of PCM and as fourth subject you need to consider the marks of an additional subject that you studied in your 10+2 except PCM.

You need to have a minimum of 60% or more marks in Mathematics. Moreover, An aggregate of 60% or more marks in one language from List A1, Mathematics and any two subjects from List B( in this case physics and chemistry).

For more information visit the following link:
https://university.careers360.com/articles/du-bsc-eligibility-criteria

Hope it helps!
ALL THE BEST!