Is it possible to have a pop up or other action when a person chooses a particular option in a drop down?

Is it possible to have a pop up or other action when a person chooses a particular option in a drop down?

Just wondering if this is possible.

I have a question on a form with 4 possible choices in drop down format.   However if the person chooses option D I would like a message to appear in some format, either pop up? or some obvious way they can see it !

many thanks