The platform gives a native tool for this situation. Use B. Set up Global Values in Picklist Value Sets. The decision turns on platform configuration. The Platform App Builder role is to choose the standard Salesforce capability that owns the behavior: data structure, access, automation, user experience, reporting, or deployment.
This is how the same requirement would be handled in a well-governed Salesforce org: identify the controlling object, relationship, field, page, automation, or environment first, and then configure the native capability for that control point. The selected answer satisfies the requirement without requiring users to perform extra interpretation or administrators to maintain an unnecessary custom workaround.
The other choices would leave a gap in the implementation. A (Establish upper bound on existing picklists in Picklist Settings.) misses the mark because That field type either stores the wrong kind of value or fails to support the required behavior as cleanly as the selected option. C (Remove upper bound on inactive picklist values in Picklist Settings.) handles a different concern; That field type either stores the wrong kind of value or fails to support the required behavior as cleanly as the selected option. D (Convert the picklist fields to a different field type that will still meet the business requirements.) is less defensible because That field type either stores the wrong kind of value or fails to support the required behavior as cleanly as the selected option.
In a real org, the builder should include negative testing as well, proving that the rule does not fire or display outside the intended conditions.
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