Which of the following device is used to connect two systems, especially if…

Concept: Networking devices operate at different layers of the OSI model, and each layer determines how much of a message a device can understand. A device that must join networks running different protocols has to read and rewrite the data all the way up to the application layer, because the two sides format, address, and encode their messages in incompatible ways. Translating between protocol stacks is therefore an upper-layer job, not a physical- or data-link-layer one.

Applying it here: Connecting two systems that speak different protocols requires full protocol conversion, so the device must operate across all OSI layers. The device that does this is the gateway: it accepts a message on one protocol stack, interprets it completely, and re-encodes it into the other side's protocol so the two otherwise-incompatible systems can communicate.

Why the other devices cannot do this:

• Hub — a physical-layer (Layer 1) device that simply repeats incoming electrical signals out of every port. It understands no addressing or protocol at all, so it cannot translate between two protocols.

• Bridge — a data-link-layer (Layer 2) device that forwards frames between segments using MAC addresses. It joins networks that already use the same protocol; it does not convert one protocol to another.

• Repeater — a physical-layer (Layer 1) device that regenerates and amplifies a weak signal to extend cable distance. Like the hub it has no awareness of protocols and performs no translation.

Cross-check: Order the candidates by OSI layer — repeater and hub at Layer 1, bridge at Layer 2 — and only the gateway reaches up through the application layer, which is exactly where protocol-to-protocol conversion must happen. This confirms the gateway is the device that connects systems using different protocols.