Answer
The sacral plexus is formed by the the anterior (ventral) rami of spinal nerves from segments L4-L5 and from S1 to S4.
Work Step by Step
Spinal nerves exit between vertebrae. This means spinal nerve S1 exits between vertebrae S1 and S2.
Some branches of the sacral plexus are the superior gluteal nerve, the sciatic, the inferior gluteal, the posterior femoral and the pudendal nerves.