∫ Each row increases from left to right, and each column increases from top to bottom respectively.

1, 2, 9 has only one filling method, and 5 can only be filled in the upper right corner or the lower left corner.

5 Fill in any of 6, 7 and 8 in the blank adjacent to it;

There is only one way for the remaining two numbers, from small to large.

* * * has 2×3=6 results,

So choose a.