The correct answer is C. CoAP.
CoAP is a lightweight, RESTful application protocol for constrained devices and low-bandwidth, low-power networks. It is designed to be an alternative to HTTP for machine-to-machine (M2M) and Internet of Things (IoT) applications.
CoAP is based on the User Datagram Protocol (UDP), which is a connectionless protocol. This means that CoAP messages are not guaranteed to be delivered, but they are typically delivered much faster than messages sent using a connection-oriented protocol like HTTP.
CoAP messages are also very small, which makes them ideal for use in constrained devices with limited memory and processing power.
CoAP is a very simple protocol, but it is still powerful enough to support a wide range of M2M and IoT applications. It is supported by a growing number of devices and platforms, and it is likely to become the de facto standard for M2M and IoT communication in the future.
Here is a brief explanation of each option:
- MQTT is a publish/subscribe messaging protocol that is often used in M2M and IoT applications. It is a more complex protocol than CoAP, but it is also more reliable.
- HTTP is a standard web protocol that is used to transfer data between clients and servers. It is a very powerful protocol, but it is not well-suited for use in constrained devices.
- SPI is a security protocol that is used to protect data in transit. It is not a communication protocol, and it is not well-suited for use in M2M and IoT applications.