The correct answer is option 3, the Saphenous vein. It is the longest vein in the human body. Let`s go through the other options for better understanding.

Option 1, the Trochlear, is actually a nerve responsible for eye movement. It is not a vein.

Option 2, the Inferior vena cava, is a large vein that carries deoxygenated blood from the lower body to the heart. While it is an important vein, it is not the longest in the human body.

Option 4, the Trigeminal, is a cranial nerve that is responsible for sensory information from the face and motor functions related to chewing. It is not a vein.

Therefore, the Saphenous vein is the correct answer as it is the longest vein in the human body, typically found in the leg.