A network admin configures a static route on the edge router of a network to assign a gateway of last resort (the router that connects to the internet/ISP) ... How would they configure the edge router to automatically share this route using RIP?

Use the default-information originate command.

Explanation:

A network administrator customizes a fixed path on a channel's edge router for creating any last recourse gateway that is the router connecting to the online / ISP. So, they using the usual command source to customize the edge router to connect this path instantly using RIP.

That's why the following answer is correct according to the scenario.