How to know which 3d model (cube) is selected

See more:

C#

WPF

I'm trying to draw 4 3D cubes in WPF.
When user click on first cube---create string (string a="abc")

When user click on second cube---create string (string b="def")

When user click on third cube---create string (string c="ghi")

When user click on fourth cube---create string (string d="klm")
Can somebody help me how to do this please.
I know that will be done using hittest...but don't know how to implement.
Please post a sample code.
Thanks