How do I force entry in multiple cell ranges in one worksheet

[CindyB]

I need to be able to force a user to choose from a drop down box from several
columns in one worksheet. As an example, I have drop down boxes in column d,
column f and column H. Rows 4 - 50 in each column.
I need to be able to force the user to choose from the drop down list if
they just tab over it to the next column, so to speak. If the cell is empty
as they 'go by', I need them to stop.
Can someone help me here?

Thanks!!