なに、だれ、いつ、どこ are marked by particle が because the speaker is asking for new information
なにがいいですか？ nani ga ii desuka? What is good? ( = what do you want?)
だれがいいですか？ dare ga ii desuka? Who is good? ( = who do you want?)
いつがいいですか？ itsu ga ii desuka? When is good? ( = when do you want?)
どこがいいですか？doko ga ii desuka? Where is good? ( = where do you want?)