Final answer:
The sciatic nerve originates from the sacral plexus, specifically from the ventral rami of L5 and S1.
Step-by-step explanation:
The sciatic nerve is a systemic nerve that comes from the sacral plexus, and it is a combination of the tibial and fibular nerves. The ventral rami that contribute to the sciatic nerve, within the sacral plexus, are from the lower lumbar nerves L4 and L5 and the sacral nerves S1 to S4. Thus, the correct answer to which ventral rami of the sacral plexus contribute to the sciatic nerve would be B) L5 and S1.