What is the longest bone in the body?

The correct answer for the longest bone in the body is the femur, option A. This is because the femur, also known as the thigh bone, runs from the hip to the knee and is the strongest and largest bone in the body. Option B, the tibia, is the second longest bone but is smaller and weaker than the femur. Option C, the humerus, is located in the upper arm and is not as long as the femur. Option D, the spine, is not a bone itself but a collection of smaller bones, making it incorrect.