Bain Capital and Apollo Global Management LLC are among several buyout firms that are still in the running to acquire a majority stake in British advertising giant WPP Plc’s data and market research unit Kantar, according to people familiar with the situation. The private equity firms are among bidders who have moved into the next round of the auction after other parties including CVC Capital Partners and Apax Partners dropped out, said the people, who asked not to be identified because discussions are private.