What prefix is used for app-level tokens, often used with Socket Mode?

Slack app-level tokens use the `xapp-` prefix. Unlike bot tokens, app-level tokens represent an application independently of an individual bot installation and are limited to specific application-level API capabilities. Socket Mode is a major use case. To establish Socket Mode, Slack requires an app-level token with the `connections:write` scope. That credential is supplied to `apps.connections.open`, which generates the temporary WebSocket URL through which Slack delivers Events API and interactive payloads. Slack's Socket Mode documentation also demonstrates storing this credential as `SLACK_APP_TOKEN` with a value beginning `xapp-`. By contrast, `xoxb-` identifies bot tokens and `xoxp-` identifies user tokens. `bearer-` is not a Slack token-prefix classification. Therefore, B is the verified answer.