Mathematics, physics, and chemistry books are stored on a library shelf that can accommodate 25 books. Currently,

The shelf can accommodate 25 books
20% of the shelf spots remain
Total books on the shelf = (100 - 20)% * 25
= 80% * 25 = 20
Let the total number of Chemistry books = x
Total number of Physics books = x + 4
Since the total no. of books = 20
Then, x + (x+6) + [12 (x+4)] = 20
2x + 4 + 2x + 8 = 20
4x + 12 = 20
Chemistry books = 2, Physics books = 6 and Mathematics books = 12
Probability of picking Mathematics book = 12/20
Probability of picking Chemistry book = 12/20
The exact order of picking the books is not known. So, for the total probability, we will estimate the sum of the following of the probability of picking Mathematics first or picking Chemistry first.

$$=\left(\frac{12}{20}\cdot\frac{2}{20}\right)+\left(\frac{2}{20}+\frac{12}{20}\right)=\frac{6}{100}+\frac{6}{100}=\frac{12}{100}$$
$$=12\%\ \ \ \ \ \ \ \ \ \ \left(Answer\ =\ option\ C\right)$$