What feature did we use to quickly apply the formatting shown in this image?

Conditional formatting.

Explanation:

Conditional Formatting is the method that main objective to enabling the users for setting the formatting to the  cell or the cell range as well as it adjust the layout based that are based on the cell value or the formula value.

The Conditional formatting making the cell value as bold as in the given question we see that the cell value is bold  when the given condition is true.The objective of pivot table is to summarizing the data it do not provide the formatting that's why this option is incorrect .The objective of what if analysis is altering the absolute value of worksheet also it see how some modifications will influence the result of the worksheet formulae in the spreadsheet it do not provide the formatting that's why this option is incorrect .Data scenario formatting do not provide the formatting that's why this option is incorrect .