The commonest cause of pulmonary embolism is -: (A) Thrombosis of leg veins, (B) Thrombosis of prostatic veins, (C) IVC thrombosis, (D) Thrombosis of internal pudendal artery
Answer is A. Thrombi in a deep vein of the leg accounts for more than 95% of cases of pulmonary embolism.
The most common source of emboli is DVT of femoropopliteal and iliac veins.
But remember that :

Most common site of DVT is calf veins
(Calf veins thrombosis are itself associated with a low risk for embolism, but when the thrombosis progresses proximally to involve the larger veins above knee, then the risk becomes very high.