Explanation:

To solve this problem, you need to find the greatest common divisor (GCD) of the numbers of books in each topic.

- Maths books: 84

- Physics books: 90

- Chemistry books: 120

1. Find the GCD: This will give you the number of books per stack.

- Prime factorization:

- 84 = 2² × 3 × 7

- 90 = 2 × 3² × 5

- 120 = 2³ × 3 × 5

- Common prime factors: 2 and 3.

- GCD = 2 × 3 = 6

- Option 1: 12

- Divides 84 but not all numbers evenly.

- Option 2: 18

- Does not divide any of the given numbers evenly.

- Option 3: 6

- GCD for all topics.

- Option 4: 21

- Does not divide any of the given numbers evenly.

Option 3 is correct.

- Answer: 6 books per stack

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