Northern Trail Outfitters wants to use Marketing Cloud to send notifications to customers when their...

Comprehensive and Detailed Explanation:

The correct answer is D — Journey Builder multi-step journey with an API entry event and a decision split.

API Entry Event: Order system triggers a real-time entry into the Journey when the order is ready.

Decision Split: Based on customer's communication preference (SMS or Email), the journey will send the appropriate message.

❌ Why others are wrong:

A: MobileConnect Outbound Message can't dynamically decide fallback channel inside AMPscript easily.

B: Transactional Send Journeys are typically for single email sends, not channel preference branching.

C: Automation Studio is time-based, not ideal for real-time order notifications and dynamic preference handling.

Exact Extract from Salesforce Official Documentation:

Salesforce Help — Journey Builder API Entry Event:

"API Event Entry allows external systems to trigger customer journeys in real-time, optionally combined with Decision Splits to deliver different channel communications based on customer attributes."

(Source: https://help.salesforce.com/s/articleView?id=sf.mc_jb_api_event_source.htm )