Final Answer:
The communication tool that enables multiple users to communicate in real time through a smartphone application or social media site is option d) Instant messaging.
Step-by-step explanation:
Instant messaging (IM) facilitates immediate, back-and-forth communication between users. Unlike blogs (option A), which are typically used for longer-form, asynchronous content, and email or webmail (options B and C), which involve more formal and often delayed communication, instant messaging is designed for instant and interactive conversations. It is particularly popular on smartphone applications and social media platforms, allowing users to exchange text messages, multimedia, and engage in group chats. This real-time nature makes instant messaging a dynamic and efficient tool for quick communication in various personal and professional contexts.
Option D is the answer.