The question requires a valid config.yaml file to set hierarchical properties used in the HTTP Listener configuration (${http.host}, ${http.port}). YAML (YAML Ain't Markup Language) uses indentation to represent structure. Option B correctly defines a top-level http key with nested host and port keys. This structure allows Mule to resolve the properties http.host and http.port correctly. The other options use incorrect syntax for a YAML file.

A. This option uses = which is typical for .properties files, not the colon (:) and indentation required by YAML syntax.

C. This option uses the key = value format of a standard Java .properties file, which is not the correct syntax for a .yaml file.

D. This option uses JSON syntax. While YAML is a superset of JSON, this is not the idiomatic or expected format for a file named config.yaml.

MuleSoft Documentation, "Configure Properties". This document illustrates how to use YAML configuration files, showing the hierarchical structure with indentation. See the section on "YAML Configuration File". (docs.mulesoft.com/mule-runtime/4.4/configuring-properties#yamlconfigurationfile)

MuleSoft Documentation, "Properties in Connectors". This page shows examples of using property placeholders like ${http.port} in connector configurations, which are then resolved from configuration files. (docs.mulesoft.com/mule-runtime/4.4/configuring-properties#properties-in-connectors)

YAML Specification (yaml.org), Version 1.2.2, Section 2.1 "Collections". The official specification defines the use of indentation and key-value pairs separated by a colon.