correct answer is: c) 2.5 kg

Explanation

Here we can use ‘Unitary Method’.
72 stories books are 9 kg
\therefore 1 story book is =\large\frac{9}{72} kg
\therefore 20 stories books are =\large\frac{9\times20}{72} kg
\rightarrow 2.5 kg
Ans: 2.5 kg.

An alternative way to phrase this particular query is possible:
1. Calculate the total weight of 20 storybooks if the weight of 72 books of the same type is 9kg.
2. What is the combined weight of 20 storybooks if the weight of 72 identical books is 9kg?
3. Determine the weight of 20 storybooks when the weight of 72 such books is known to be 9kg.
4. Calculate the weight of 20 storybooks given that the weight of 72 books of the same kind is 9kg.
5. Find the total weight of 20 storybooks if the weight of 72 similar books is 9kg.