In addition to the previous excellent responses, if you have a newer version
of PowerPoint you can use "Triggers". This would allow you to have two text
boxes that are set to appear when they click on either the "True" button or
the "False" button. You can have the desired text in each box.
Of course if you REALLY want to get fancy you can use VBA to program
PowerPoint to do some of this stuff. I have a simple 10 question review
presentation that you can download and are free to use. On those you can
delete two of the incorrect choices and be left with a "Correct" and
"Incorrect" box which you can use as True/False questions instead. The code
is open on that file so you can see how it was done.
The link is:
http://www.pttinc.com/cbt_development.html