When designing a network, which method can be used to control the exit point for traffic leaving an autonomous system, at the Layer 3 control plane?

BGP best-path selection uses the IGP metric to the BGP next-hop as a tiebreaker when other attributes are equal, so by increasing or decreasing the IGP cost toward a specific ASBR, operators can steer outbound traffic to a preferred exit point. This is a pure Layer 3 control plane mechanism operating through the routing protocol itself, without requiring BGP policy changes or coordination with neighboring ASes.

MED is sent to neighboring ASes to influence how they route traffic inbound toward your AS; it signals preferred entry points to external peers and has no effect on which exit point your own AS uses for outbound traffic.

Site of Origin is an MPLS VPN extended community used to prevent routing loops when CE sites have backdoor connections between them, and it is not a mechanism for controlling outbound exit points from an AS.

AS path prepending increases the apparent AS path length advertised to neighboring ASes, making your prefixes less attractive to them for inbound routing; it has no bearing on which exit point your own AS selects for traffic it originates or transits.