Let's assume that total no. of students in the school is x
Also , we are given that their are 140 students in 6th grade {Given} and no. of students in 6th grade is 40% of total no. of students in school i.e 40% of x {From Pie Chart} . Mathematically can be written as ;
After cancelling ;
So , Total population of school is 350
Hence, Option D) 350 is correct