Slack's documented macOS and Linux installation method uses the official shell installer hosted on Slack's downloads domain. The command `curl -fsSL https://downloads.slack-edge.com/slack-cli/install.sh | bash` retrieves the installer securely and runs it through Bash, installing the Slack CLI and making the `slack` command available for development. Slack's current CLI installation documentation presents this command as the supported installation path for macOS and Linux. The alternatives do not represent the official installation command in the Slack Developer documentation: Slack CLI is not installed through the listed npm or pip commands, and `brew install slack` does not correspond to Slack's documented CLI installer. Therefore, option B is the verified installation command for the environments named in the question.