Direct Naked Single

A direct naked single is a method of determining the digit to fill in a cell by examining each empty cell and its peer cells one by one.

For example, if we examine each empty cell in the grid below one by one, then when we reach $R5C4$, we can see that digits $1$, $3$, $4$, $5$, $6$, $7$, $8$, and $9$ have already appeared in its peer cells. According to the rules of Sudoku, the only remaining digit that can be placed in $R5C4$ is $2$: