Answer: Option C
:
C
Let x dl of pure milk be added. The concentration of pure milk is 100%
Total milk content in x dl of milk = x * 100% = x dl
Total milk content in 800 dl of 15% mixture = 800 * 15% = 120 dl
Therefore, total milk content in final mixture = x + 120
Total volume of mixture = 800 + x
Hence concentration of milk in final mixture = $\frac{x+120}{800+x}$
This is given to be 32%.
Hence$\frac{x+120}{800+x}$ = 32 %
Solving, we get x = 200 dl.
Shortcut- Using Alligation
There may be different ways to solve this question, but one of the fastest approaches is using the concept of Alligation. Pure milk has 100% concentration, and it is mixed with a solution with 15% milk to make it 32%. This information can be used to get the ratio of the mixture as follows

For every 4 parts of 15% milk, 1 parts of 100% milk needs to be added.
Hence to 800 dl of the 15% solution, $\frac{800}{4}$ = 200 dl of 100% milk needs to be added