In the diagram above each of the letters represents an odd single-digi

By definition a prime number is divisible only by itself and "1".
This does not apply to "1",so 1 is not considered a prime number.

HarshavardhanR how do you even get 9 distinct odd primes all single digit? there are only 3!!
"each row and each column must be filled with distinct digits" what am I missing?

[color=#0f0f0f]In the diagram above each of the letters represents an odd single-digit prime.

- There are only 3 odd single-digit primes: 3,5,7.
- So, each letter in the diagram is represented by either a 3 or a 5 or a 7.

If each row and each column must be filled with distinct digits

- Each row has distinct digits i.e., if you take any one row, there will be no repetition of digits.
- Each column has distinct digits i.e., if you take any one column, there will be no repetition of digits.

Here is one example ->
[/color]

7	3	5
5	7	3
3	5	7

Observe - each row, whether you take row 1 or row 2 or row 3 - has distinct digits (each row has 3, 5, and 7 - but no repeat of any digit).
Similarly, each column, whether you take column 1 or column 2 or column 3 - has distinct digits (each column has 3, 5, and 7 - but no repeat of any of the digits).

This is actually similar to a Sudoku Grid - every row and every column has distinct digits.

Hope this helps!

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Harsha