To solve this problem, we must determine the order of the 10 books, adhering to the given conditions. We have:

• 3 Geography books (G)
• 3 Hindi books (H)
• 2 Mathematics books (M)
• 2 Economics books (E)

Here are the constraints provided:

1. There is an Economics book between a Geography and a Mathematics book. (G, E, M)
2. There is a Geography book between a Mathematics and an Economics book. (M, G, E)
3. There is a Hindi book between an Economics and a Mathematics book. (E, H, M)
4. There is a Mathematics book between two Hindi books. (H, M, H)
5. There are two Hindi books between a Mathematics and a Geography book. (M, H, H, G)

Let's arrange the books step-by-step:

• From constraint 4: H, M, H.
• From constraint 5: M, H, H, G.
• To satisfy both, place them as: M, H, M, H, G.
• Now, integrate the remaining constraints into this setup:

Working with the rest of the constraints:

• From constraint 1: Choose (M, G) from existing and place E between them: M, G, E. One possible arrangement already supporting constraint 2 and 1 is: E, G, M.
• Add in Hindi and Economics, starting from the remaining Geography: M, H, M, H, G, E, G, E.
• The chronological sequence of books becomes:
  1. E
  2. G
  3. M
  4. H
  5. M
  6. H
  7. E
  8. G
  9. ......(two remaining Hindi and Economics)

According to the arrangement, the sixth book is "Hindi." Hence, the correct answer is Hindi.