Which of the following devices understands the format and contents of the data…

Concept: A device's position in the OSI model decides how much of a message it can understand. A device working at the application layer can read the meaning and format of the data itself; devices working at the physical, data-link, or network layers only move bits, frames, or packets without understanding what the payload actually contains.

Application: The question describes exactly this ability — understanding a message's format and translating it into a different format. Gateways are protocol converters, and the application gateway — the type built for message-format translation — operates at the application layer: it reads a message's content and converts it between different protocols or formats, for example translating a message between two different email-system formats. That is precisely the behaviour asked for, so gateway is the answer.

Why the other devices don't fit:

• Hub — works at the physical layer; it only repeats the incoming electrical or optical signal out to every other port and has no awareness of data format at all.

• Switch — works mainly at the data-link layer; it forwards frames to the correct port using MAC addresses, without reading or understanding the payload's format.

• Router — works at the network layer; it forwards packets using IP addresses and can even rewrite addresses through NAT, but that address-level work is not the same as reading and translating the application data format.