The correct answer is option 3 - Femur. The femur is the longest bone in the human body, located in the thigh region. It extends from the hip joint to the knee joint, making it the primary bone involved in supporting body weight and allowing for movements like walking and running.

Option 1 - Ulna, is one of the two bones in the forearm and is not as long as the femur.

Option 2 - Humerus, is the bone in the upper arm and is also shorter than the femur.

Option 4 - Tibia, is the larger bone in the lower leg, but it is not as long as the femur either.

Therefore, the longest bone in the human body is the femur.