Hidden Single
Easy
Used by Smart Hint
A digit has only one possible cell left in a row, column, or box, even though that cell has other candidates too.
- Where the conclusion lands
How it works
- Scan this box for digit 7: only D4 still lists it as a candidate — every other empty cell has ruled 7 out.
- D4 has other candidates too, so it isn't a Naked Single — but since 7 has nowhere else to go in the box, it's a Hidden Single. Place 7 there.