What part of the femur articulates with the tibia to form the knee joint?

The patella articulates with the tibia to form the knee joint.

Explanation:

Patella is called the knee cap and it gives protection to the knee.  Femur is the longest bone in the body and has the function of supporting the weight of the body and enhancing the movement of the legs. The knee joint permits the extension of the lower leg relative to the thigh.

It allows a 120 degree movement unlike other joints that allow lesser freedom of movement. Patella is the part of the femur that articulates with the tibia and forms the knee joint.