NEW YORK (Reuters) - U.S. President-elect Donald Trump has offered retired Lieutenant General Michael Flynn the position of White House national security adviser, a senior Trump official said on Thursday. Asked if Flynn is expected to accept, a person familiar with the offer replied: “When the president of the United States asks you to serve, there is only one answer.” Flynn, a former head of the Defense Intelligence Agency, advised Trump during the campaign on national security issues and often served as an introductory speaker at campaign rallies. 