How would you say the following questions:
What/which Mac OS X version do you use?
What/which graphic design software will you use?
?
I have read "Which" vs. "what" — what's the difference and when should you use one or the other? and the remark of the main anwer:
requires a choice between a number of items.
but here it seems to be an implicit choice between a few popular items (such as Photoshop, GIMP, MSPaint, etc.)
The answer at "Which" vs. "what" — what's the difference and when should you use one or the other? by sirmirzo says
We usually use which when we are asking about a fixed or limited number of things or people, and what when we are not. Often, however, we can use either which or what with little difference in meaning.
Note that (i) "usually" and "often" do not mean "always" and that (ii) the wording allows you some discretion.
There are no rules in English: there is guidance. Some guidance looks like a rule, it is not.
I think you should use which only when your audience is aware of the finite set of possibilities. For example, if I have three kinds of ice cream in the freezer, I would ask "which flavor would you like?" If I'm going to the store to buy ice cream, I would ask, "what flavor should I buy?"
In your case, if there are only three graphic design programs available in your office, it's probably best to use which. If users are free to install whatever program they want, you should probably use what.
