In a puzzle of filling a grid, each row and column in the 9x9 grid, as well as each 3x3 sub-grid shown with heavy borders, must contain all the digits 1-9.

In the above partially filled grid, the number 3 appears in square marked

Correct option is A

Solution:
Understanding the Sudoku Rules:

Each row and column must contain digits 1 to 9 exactly once.
Each 3×3 sub-grid (with heavy borders) must also contain digits 1 to 9 exactly once.
Focusing on the 3×3 sub-grid containing A, B, C, and D:

The grid already has some numbers filled in.
The missing numbers in the top-right 3×3 sub-grid must be placed correctly according to Sudoku rules.
Finding the correct placement for 3:

Checking the existing 3s in the grid, we see their positions in other 3×3 sub-grids.
The only valid placement for 3 in the given 3×3 sub-grid is in square D.
Final Answer:
Thus, the number 3 appears in square D, making option (a) D the correct answer.