Which format is commonly used for Slack app manifests?

Slack app manifests are officially expressed in JSON or YAML. Both formats represent the same structured application configuration and can define metadata, application features, OAuth configuration, event subscriptions, interactivity settings, and other supported manifest properties. Slack's manifest documentation explicitly states that manifests are reusable configurations expressed in JSON or YAML and can be authored through an application's configuration page. Slack's Deno SDK also supports defining compatible manifest information programmatically in TypeScript, but among the supplied answers, JSON or YAML is unequivocally correct for standard app manifests. XML, INI, and Markdown are not the documented manifest serialization formats. Using JSON or YAML also makes manifests practical for source-control workflows, code review, automated deployment, and reuse across related application configurations. Therefore, A is the verified answer.