Randomizing letters

So I am a psychologist trying to create a simple program.

How can I make a randomized specific letters to show up at the centre of the page?

Something along the lines of “Randomize (D,E,S,T) so that only 1 of these letters show up at the center”

Background information so that you may get an idea of the program I am making:
Trying to essentially create a bunch of slides so that it runs from slide 1-6 then repeat.
Slide 2 and 6 are the “target” and “experiment” slides where a letter appears in slide 2 but when the sequence of slides repeat, slide 2 is constantly shows a letter from the randomized pool of selected letters.

func random_letter(list as string) as string
dim r as new random
return mid(list,x,1)