Solution

MQTT (Message Queuing Telemetry Transport) is the most commonly used protocol for IoT devices due to its lightweight nature and ability to work efficiently in constrained environments with limited bandwidth. It is designed for low-power devices and low-latency communication, making it ideal for IoT applications where devices may have limited computing power and unstable network conditions. Why Other Options are Wrong: a) HTTP is heavier and not optimized for the low-bandwidth, high-latency environments typical of IoT devices. b) FTP (File Transfer Protocol) is used for transferring files and is not suitable for real-time communication in IoT. d) SMTP is a protocol used for sending emails, not for device communication in IoT. e) SNMP (Simple Network Management Protocol) is used for network management, not for lightweight IoT communication.