Which flight was hijacked and crashed into the Pentagon?

The flight that was hijacked and crashed into the Pentagon is American Airlines Flight 77. This flight was part of a coordinated terrorist attack on September 11, 2001, where hijackers took control and diverted the aircraft towards Washington D.C. It ultimately crashed into the western side of the Pentagon, resulting in significant destruction and loss of life. This tragic event was part of a larger series of attacks that day, which also included the crashing of flights into the World Trade Center.

Other flights mentioned in the options were involved in different circumstances or did not involve the Pentagon. For instance, United Airlines Flight 93 was hijacked but crashed in Pennsylvania after passengers attempted to regain control from the hijackers. The other two choices, Delta Airlines Flight 123 and Southwest Airlines Flight 456, did not have any involvement in the hijackings or events of September 11, 2001. Thus, American Airlines Flight 77 is correctly identified as the specific flight that impacted the Pentagon during these events.