You're a developer who is creating an app that posts messages, and you need to...

Public channels are the only conversation type among these choices for which Slack provides an explicit permission enabling an app to post without first becoming a member. The relevant permission is the chat:write.public scope, used together with chat:write. Slack describes chat:write.public as permitting a Slack app to send messages to public channels in which the application is not a member. Without that additional scope, an app normally must join the public channel or be invited before posting. Private channels require appropriate membership and authorization because allowing an app to post into a private conversation without membership would violate Slack's privacy model. Likewise, ordinary direct messages and multi-person direct messages have different membership and conversation-opening semantics and are not equivalent to posting freely into a public channel. The wording “by default” in the source is somewhat imprecise because granular-permission Slack apps do not automatically receive access to all public channels. They must request chat:write.public. Nevertheless, among the four conversation types, D represents the intended and technically supported capability. This question aligns with Design Your Workflows: Spotlight on Slack's APIs, particularly chat.postMessage, conversation membership, and OAuth scope behavior.