Naked Single

Easy Used by Smart Hint

Only one candidate height remains for this cell — place it.

Cells that establish the pattern
Where the conclusion lands

How it works

Heights 1 to 4 each appear exactly once in every row and every column — the same rule Sudoku's rows and columns follow, just without the boxes.
This row already shows a 2, a 4 and a 1. Only the 3 is unaccounted for.
One candidate left in the cell makes it a Naked Single. Place the 3 — no clue needed at all.