Step 2 · Easy
What is Naked Single (only one number fits)? How to Solve Sudoku
In short
When a cell has only one candidate left after crossing out the digits in its row, column, and box, that candidate is its answer.
What it is
Pick a cell and cross out every digit already in its row, column, and box. If only one digit is left, that's the answer.
Example: the row has 1–5, the column has 6 and 7, and the box has 8. Only 9 can go there.
How to spot it
Look where busy rows, columns, and boxes cross. Those cells usually have the fewest options.
Example
Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the blue digit is the answer.
R1C8 can only be 6. Already used: 1, 3, 5, 7, and 9 in its row; 1, 5, and 9 in its column; 1, 2, 4, 5, 8, and 9 in its box. Of 1–9, only 6 is left.
→ R1C8 is 6.