Option C is correct because adjusting the ruleset.xml file with the new rule is the simplest and most effective way to implement the new code rule. The ruleset.xml file defines the coding standards that are applied by PHP_CodeSniffer. By extending the Magento 2 Coding Standard and adding a new rule, the Architect can customize the code analysis and enforce the company coding standards. The new rule can use the Magento2.Namespaces.ForbiddenNamespaces sniff to check for any usage of Helper classes in the code and report them as errors or warnings1.

Option A is incorrect because creating a new composer package under the AwesomeAgency\CodingStandard\ namespace is not enough to implement the new code rule. The composer package is just a way to distribute and install the coding standard, but it does not define the rules themselves. The Architect still needs to create a ruleset.xml file and register it with PHP_CodeSniffer2.

Option B is incorrect because creating a new class \AwesomeAgency\CodingStandard\Ruleset\ForbiddenNamespaces and specifying the rule inside the process method is unnecessary and complicated. The Architect does not need to create a new class or a new sniff for this rule, as there is already an existing sniff in the Magento 2 Coding Standard that can be used for this purpose. The Magento2.Namespaces.ForbiddenNamespaces sniff can be configured with an include-pattern element to specify which namespaces are forbidden1.

References:

1: Magento 2 Coding Standards | Adobe Commerce Developer Guide

2: How to create a custom coding standard | PHP_CodeSniffer Documentation