Comprehensive and Detailed Explanation:

The correct answer is B — Use the Transactional Messaging API.

Salesforce Marketing Cloud offers the Transactional Messaging API specifically for sending critical, real-time, event-triggered communications like account logins, security alerts, order confirmations, etc.

These messages are delivered independently of Marketing Cloud’s normal send queues, ensuring faster delivery.

Transactional API messages are meant for one-to-one, personalized communications requiring immediate sending.

❌ Why others are wrong:

A: Injecting into a Journey is slower compared to the Transactional API and not optimized for high-priority transactional messages.

C: Triggered Email Interactions are an older feature and do not provide the scalability or deliverability speed of the Transactional Messaging API.

D: User-Initiated Emails are typically used for larger batches, not for individual transactional notifications.

Exact Extract from Salesforce Official Documentation:

Salesforce Help — Transactional Messaging API Overview:

"The Transactional Messaging API sends timely, personalized, high-priority messages. These messages are initiated from a system event, such as a user logging in from a new IP address."

(Source: https://help.salesforce.com/s/articleView?id=sf.mc-transactional-api.htm )

—