Sure, I can help you with that!

Here are the steps to bold only one gridline in an Excel graph:

1. Select the chart by clicking on it.
2. Click on the gridline that you want to bold. In this case, it would be the gridline that intersects the 100% line.
3. Right-click on the gridline and select "Format Gridlines" from the drop-down menu.
4. In the "Format Gridlines" pane that appears on the right-hand side of the screen, select the "Line Color" tab.
5. Under "Line Color," select the drop-down menu next to "Weight" and choose a thicker line weight. This will make the gridline bold.
6. Click "Close" to exit the "Format Gridlines" pane.

And that's it! The gridline that intersects the 100% line should now be bolded. Let me know if you have any other questions!