如果定义域用D表示,那么值域应该用什么字母表示?

Usually,D stands for domain,R stands for range.
But other notations can be used if you have clarified them in your article ahead of time.
It's true that some notations are so classic that almost all mathematician will use them.
But when your article is too long and you use them up,you can define new notations by yourself.
So this question is not essential in Mathematics.