You can make an array of your string value and pick the random value from array.
// String array like in below format.
string [] stringArray = new [] {"A", "B", "C", "D", "E", "F"}; // Make it accordingly
int ramdomNum = random.Next(0, stringArray.Length);
Console.Write(stringArray[ramdomNum]);
Hope this will help you.
solved How can I randomly select from prewritten strings in C#? [closed]