The correct answer is C.
Khalid Sheikh Mohammed is a Pakistani Islamist militant and a former member of Osama bin Laden's Al-Qaeda organization. During his interrogation at the Guantanamo Bay detention camp he confessed to his role and participation in the most significant terrorist plots all over the world.
He also confessed to masterminding the September 11 attracts. He first presented the idea of the September 11 plot to Osama bin Laden in 1996 in Afganistan. He envisioned the hijacking of twelve airplanes on both the East and the West coast to be crashed into various targets, some of them being: the World Trade Centre, the Empire State Building, the Pentagon, the White House, the Sears Tower, the Columbia Centre and some more.