The company that runs the biggest institution of higher education in the United States announced late Monday that it had received a subpoena from federal investigators who are looking into its practice of awarding grants for stock options. The company, the Apollo Group, which is the parent of the University of Phoenix, previously reported that it had hired an outside firm to review its practices, after it was identified in news articles as one of several companies that may have back-dated such grants. Apollo officials have said they acted appropriately.