How do you adjust transparency in ArcMap?

To adjust the transparency of a layer in ArcMap, you'll use the Effects toolbar. Here's how:

1. Click Customize on the main menu.
2. Select Toolbars from the dropdown.
3. Choose Effects. The Effects toolbar will appear.
4. In the Effects toolbar, click the layer dropdown arrow and select the layer you wish to make transparent.
5. Use the slider bar next to the "Transparency" label to adjust the transparency level. Sliding it to the right increases transparency, making the layer more see-through.

Essentially, by manipulating the transparency slider on the Effects toolbar, you control how much the underlying layers are visible through the selected layer.