As General Motors Co. tries to hammer out a deal to sell its long-struggling Europe business to Peugeot owner PSA Group, one multibillion-dollar issue looms large: how the companies will manage an underfunded pension plan for retirees. GM’s pension plan for Opel and Vauxhall retirees is underfunded by about $9 billion, according to data compiled by Bloomberg News. PSA would like GM to keep a big chunk of that responsibility, if not all of it, said two people familiar with the discussions, who asked not to be named because talks are private.