Mike: Sure. I have two pens. This pen has black ink. That pen has red ink. Which do you want?
TDOL: That one. Thanks.
One student asked me if she can use "what do you want" here. I don't see any semantically wrong. I'd say it's better to use "which" in this context because Mike has two pens for TDOL to choose.
Am I right?
"Which" would be better for two reasons. First, the choice is bewteen a finitie number of identified items. Second, without using the noun (pen) or a pronoun (such as one), "What do you want" can mean something totally different than a choice of pens.
Which (pen/one) do you want. -- the best
What pen do you want. -- marginal
What do you want. -- poor