Which solution should they use?

Northern Trail Outfitters has a customer login page on their website and wants to use Marketing Cloud to send an email to customers who need to reset their password. The email needs to be sent as quickly as possible. Their web server should receive a notification when the email is sent.

Which solution should they use?
A . Triggered Send Definition
B . Transactional Messaging API
C . Multi-Step journey with an API Event entry source
D . Single Send journey with an API Event entry source

Answer: A