答案：B

选B　考查情态动词。句意：父母们在做他们力所能及的事来帮助他们的孩子实现他们的夙愿。完整形式为“Parents are doing all they can do to help their children achieve their ambitions.”其中they can do是定语从句，修饰先行词all; to help  their children achieve their ambitions是目的状语。由于主句中出现了实义动词doing，所以省略了从句中的do。故答案为B。